The engines are different, and I imagine that Gdata is built for 4.
Also, judging by igor's response:
As for why not ending the support for 4.8... well, all of the existing pre-2012 editions of g-data would stop working (or more precisely, updating), for example, right?
That suggests to me, that the newer versions may use the newer engine? (igor?)
Either way I can't see how this is down to avast, since it is Gdata that is using it, they are responsible for updating...
You choose to use Gdata, you have to accept what comes with it.