It could be a market share type of thing. Avast 4.x is the last of the classic clients that runs on Win95, Win98, NT 4.0.
If there are a lot of people still using the paid version of Avast 4, they may be required/obligated to fullfill the end of subscription expirations. I can confirm as recently as last week that 4.8 continues to receive updates.
IMO, I don't run AV on any of my Windows 95 systems. I've never gotten viruses and the hardware (120MHz, 40MB RAM) is probably too slow to run the malware, LOL.