diff --git a/README.md b/README.md
index 69da535..35c3272 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,9 @@ Highlights:
 - Comes with standard Debian package, easy to manage and automate
 - **No JavaScript is involved** in the whole process, as I believe JavaScript is harmful to developers
 
-## Installation / Usage
+## Usage
+
+### Installation
 
 1. [Download the latest release](https://github.com/Jamesits/pve-fake-subscription/releases/latest)
 1. Install: run `dpkg -i pve-fake-subscription_*.deb` as root on every node
@@ -38,7 +40,7 @@ The fake subscription status doesn't grant you free access to the enterprise rep
 - [Proxmox Mail Gateway (PMG)](https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html#pmg_package_repositories)
 - [Proxmox Backup Server (PBS)](https://pbs.proxmox.com/docs/installation.html#proxmox-backup-no-subscription-repository)
 
-## Uninstallation
+### Uninstallation
 
 Run as root:
 
@@ -48,10 +50,45 @@ apt purge pve-fake-subscription
 
 This will revert your system to a "no subscription key" status.
 
-## Building the Package
+## Development Notes
+
+### Building the Package
 
 Install [nFPM](https://nfpm.goreleaser.com/install/), then:
 
 ```shell
 ./package.sh
 ```
+
+### Compatibility for Old Proxmox VE Versions
+
+#### PVE 4.x
+
+PVE 4.x is supported with minor changes to the script.
+
+Changes needed:
+- License key needs to be changed from `pve8p` to `pve4p`
+
+#### PVE 3.x
+
+PVE 3.x is supported with minor changes to the script.
+
+Changes needed:
+- The script's hashbang need to be changed from `#!/usr/bin/env python3` to `#!/usr/bin/env python`
+- License key needs to be changed from `pve8p` to `pve4p`
+
+Installation with `dpkg -i` will not work. Use the following script to install manually:
+```shell
+mkdir -p /tmp/pve-fake-subscription
+dpkg-deb -x pve-fake-subscription_*.deb /tmp/pve-fake-subscription
+sed -i'' -e's/python3/python/g' -e's/pve8p/pve4p/g' /tmp/pve-fake-subscription/usr/bin/pve-fake-subscription
+mv /tmp/pve-fake-subscription/usr/bin/pve-fake-subscription /usr/local/bin/
+rm -rf /tmp/pve-fake-subscription
+ln -sf /usr/local/bin/pve-fake-subscription /etc/cron.hourly/pve-fake-subscription
+/usr/local/bin/pve-fake-subscription
+```
+
+Removal:
+```shell
+rm -f /usr/local/bin/pve-fake-subscription /etc/cron.hourly/pve-fake-subscription
+```
diff --git a/nfpm.yaml b/nfpm.yaml
index 1f333a2..427ed53 100644
--- a/nfpm.yaml
+++ b/nfpm.yaml
@@ -1,7 +1,7 @@
 name: "pve-fake-subscription"
 arch: "all"
 platform: "linux"
-version: "0.0.9"
+version: "0.0.10"
 version_schema: "semver"
 version_metadata: "git"
 
diff --git a/scripts/postinst b/scripts/postinst
index a39394a..da7c9c6 100755
--- a/scripts/postinst
+++ b/scripts/postinst
@@ -3,15 +3,19 @@
 after_upgrade() {
     :
 
-    systemctl --system daemon-reload >/dev/null || true
-    debsystemctl=$(command -v deb-systemd-invoke || echo systemctl)
-    if ! systemctl is-enabled pve-fake-subscription.timer >/dev/null
-    then
-    : # Ensure this if-clause is not empty. If it were empty, and we had an 'else', then it is an error in shell syntax
-        systemctl preset pve-fake-subscription.timer >/dev/null || true
-        $debsystemctl start pve-fake-subscription.timer >/dev/null || true
+    if command -v systemctl >/dev/null; then
+        systemctl --system daemon-reload >/dev/null || true
+        debsystemctl=$(command -v deb-systemd-invoke || echo systemctl)
+        if ! systemctl is-enabled pve-fake-subscription.timer >/dev/null
+        then
+        : # Ensure this if-clause is not empty. If it were empty, and we had an 'else', then it is an error in shell syntax
+            systemctl preset pve-fake-subscription.timer >/dev/null || true
+            $debsystemctl start pve-fake-subscription.timer >/dev/null || true
+        else
+            $debsystemctl restart pve-fake-subscription.timer >/dev/null || true
+        fi
     else
-        $debsystemctl restart pve-fake-subscription.timer >/dev/null || true
+        ln -sf /usr/bin/pve-fake-subscription /etc/cron.hourly/pve-fake-subscription
     fi
 
     pve-fake-subscription
@@ -20,10 +24,14 @@ after_upgrade() {
 after_install() {
     :
 
-    systemctl --system daemon-reload >/dev/null || true
-    debsystemctl=$(command -v deb-systemd-invoke || echo systemctl)
-    systemctl preset pve-fake-subscription.timer >/dev/null || true
-    $debsystemctl start pve-fake-subscription.timer >/dev/null || true
+    if command -v systemctl >/dev/null; then
+        systemctl --system daemon-reload >/dev/null || true
+        debsystemctl=$(command -v deb-systemd-invoke || echo systemctl)
+        systemctl preset pve-fake-subscription.timer >/dev/null || true
+        $debsystemctl start pve-fake-subscription.timer >/dev/null || true
+    else
+        ln -sf /usr/bin/pve-fake-subscription /etc/cron.hourly/pve-fake-subscription
+    fi
 
     pve-fake-subscription
 }
diff --git a/scripts/postrm b/scripts/postrm
index eab36b4..77de063 100755
--- a/scripts/postrm
+++ b/scripts/postrm
@@ -6,6 +6,8 @@ after_remove() {
     rm -f /etc/subscription
     rm -f /etc/pmg/subscription
     rm -f /etc/proxmox-backup/subscription
+
+    rm -f /etc/cron.hourly/pve-fake-subscription
 }
 
 after_purge() {
diff --git a/usr/bin/pve-fake-subscription b/usr/bin/pve-fake-subscription
index 8deeaa3..f94a437 100755
--- a/usr/bin/pve-fake-subscription
+++ b/usr/bin/pve-fake-subscription
@@ -3,6 +3,8 @@
 # Pollute Proxmox software subscription cache so it won't alert you on dashboard login
 # Should be scheduled to run every few hours with a timer to prevent cache from expiring
 # If you need to prevent it checking keys against a server, please block "shop.maurer-it.com" in your hosts file
+
+from __future__ import print_function
 import hashlib
 import base64
 import json
@@ -10,7 +12,6 @@ import time
 import re
 import sys
 import os
-from typing import List
 from datetime import datetime, timedelta
 
 # PVE & PMG: /usr/share/perl5/PVE/Subscription.pm
@@ -18,30 +19,40 @@ from datetime import datetime, timedelta
 shared_key_data = "kjfdlskfhiuewhfk947368"
 server_key_file = "/etc/ssh/ssh_host_rsa_key.pub"
 
-def get_timestamp() -> int:
+# Default license keys
+lic_pve = "pve8p-1145141919"
+lic_pmg = "pmgp-1145141919"
+lic_pbs = "pbst-1145141919"
+
+# UI customization
+ui_product_name = "YajuuSenpai"
+ui_message = "Yajuu Senpai has got your back"
+ui_url = "https://github.com/Jamesits/pve-fake-subscription"
+
+def get_timestamp():
     return int(time.time())
 
 # Perl's md5_base64 implementation
-def md5_base64_perl(x: str) -> str:
+def md5_base64_perl(x):
     return base64.b64encode(hashlib.md5(x.encode()).digest()).strip(b'=').decode()
 
 # Rust's `base64::encode(tools::md5sum("something")?);`
-def md5_base64_rs(x: str) -> str:
+def md5_base64_rs(x):
     return base64.b64encode(hashlib.md5(x.encode()).digest()).decode()
 
-def generate_server_id(key: str) -> str:
+def generate_server_id(key):
     return hashlib.md5(key.encode()).hexdigest().upper()
 
-def dt_string(format: str, offset_secs: int = 0) -> str:
+def dt_string(format, offset_secs=0):
     return (datetime.now() + timedelta(seconds=offset_secs)).strftime(format)
 
-def generate_subscription_pve_pmg(key: str, server_ids: List[str]) -> str:
+def generate_subscription_pve_pmg(key, server_ids, product_name=ui_product_name):
     localinfo = {
         "checktime": get_timestamp(),
         "status": "Active",
         "key": key,
         "validdirectory": ",".join(server_ids),
-        "productname": "YajuuSenpai",
+        "productname": product_name,
         "regdate": dt_string("%Y-%m-%d %H:%M:%S"),
         "nextduedate": dt_string("%Y-%m-%d", 1296000),
     }
@@ -53,9 +64,10 @@ def generate_subscription_pve_pmg(key: str, server_ids: List[str]) -> str:
     return key + "\n" + csum + "\n" + data + "\n"
 
 # key_pattern can be find in /usr/share/perl5/{PVE,PMG}/API2/Subscription.pm
-# PVE: r'pve([1248])([cbsp])-[0-9a-f]{10}'
+# PVE5+: r'pve([1248])([cbsp])-[0-9a-f]{10}'
+# PVE3/4: r'pve([124])([cbsp])-[0-9a-f]{10}'
 # PMG: r'pmg([cbsp])-[0-9a-f]{10}'
-def activate_pve_pmg(key: str, subscription_file: str) -> None:
+def activate_pve_pmg(key, subscription_file, *args, **kwargs):
     # check if the key format is correct
     # pattern = re.compile(key_pattern)
     # if not pattern.match(key):
@@ -68,23 +80,23 @@ def activate_pve_pmg(key: str, subscription_file: str) -> None:
         server_id = generate_server_id(f.read())
 
     # generate a license file
-    subscription = generate_subscription_pve_pmg(key, [server_id])
+    subscription = generate_subscription_pve_pmg(key, [server_id], *args, **kwargs)
 
     # write license file
     with open(subscription_file, "w") as f:
         f.write(subscription)
 
-def generate_subscription_pbs(key: str, server_ids: List[str]) -> str:
+def generate_subscription_pbs(key, server_ids, product_name=ui_product_name, message=ui_message):
     localinfo = {
         "status": "active", # PBS: `new`, `notfound`, `active`, `invalid`
         "serverid": ",".join(server_ids),
         "checktime": get_timestamp(),
         "key": key,
-        "message": "Yajuu Senpai has got your back",
-        "productname": "YajuuSenpai",
+        "message": message,
+        "productname": product_name,
         "regdate": dt_string("%Y-%m-%d %H:%M:%S"),
         "nextduedate": dt_string("%Y-%m-%d", 1296000), # 1296000: MAX_LOCAL_KEY_AGE in src/tools/subscription.rs
-        "url": "https://github.com/Jamesits/pve-fake-subscription",
+        "url": url,
     }
 
     data = base64.standard_b64encode(json.dumps(localinfo).encode()).decode()
@@ -94,14 +106,14 @@ def generate_subscription_pbs(key: str, server_ids: List[str]) -> str:
     return key + "\n" + csum + "\n" + data + "\n"
 
 # Key pattern: pbst-xxxxxxxxxx
-def activate_pbs(key: str, subscription_file: str) -> None:
+def activate_pbs(key, subscription_file, *args, **kwargs):
     # get machine ID
     server_id = ""
     with open(server_key_file, "r") as f:
         server_id = generate_server_id(f.read())
 
     # generate a license file
-    subscription = generate_subscription_pbs(key, [server_id])
+    subscription = generate_subscription_pbs(key, [server_id], *args, **kwargs)
 
     # write license file
     with open(subscription_file, "w") as f:
@@ -111,14 +123,14 @@ if __name__ == "__main__":
     # Proxmox VE
     if os.path.exists("/etc/pve"):
         print("Activating Proxmox VE...")
-        activate_pve_pmg("pve8p-1145141919", "/etc/subscription")
+        activate_pve_pmg(lic_pve, "/etc/subscription")
 
     # Proxmox Mail Gateway
     if os.path.exists("/etc/pmg"):
         print("Activating Proxmox Mail Gateway...")
-        activate_pve_pmg("pmgp-1145141919", "/etc/pmg/subscription")
+        activate_pve_pmg(lic_pmg, "/etc/pmg/subscription")
 
     # Proxmox Backup Server
     if os.path.exists("/etc/proxmox-backup"):
         print("Activating Proxmox Backup Server...")
-        activate_pbs("pbst-1145141919", "/etc/proxmox-backup/subscription")
+        activate_pbs(lic_pbs, "/etc/proxmox-backup/subscription")