I think the reason might be is 4.8 works on windows 7. Windows 7 still has security updates from Microsoft, and they would be doing so until January 14, 2020. Any windows 7 user with Avast 4.8 would be unsecured if Avast stopped updating 4.8 now.
Probably by the time windows 7 support end, the current version of avast then isn't compatible with windows xp at all and the majority of the people would be running Windows 11, 12 or whatever MS is going to call windows 10 successor.
I usually update to the latest version of Avast at least a month after it's released so I don't have to deal with bugs and stuff.