AutoIt. Just have it play a Windows ding sound every 4min, but also set the volume to zero first.
Have the script launch at login/startup.
AutoIt doesn't require an install, you can simply copy the script engine to a folder with your script. Been a while, but you may be able to compile your script with the engine too, so you have a stand-alone script.
Or make a silent, 1/4 second long wav/mp3, and use that to ensure you don't hear anything.