That is because there are no further program updates for avast 4.8, it is effectively on life support. It hasn't had a program update in some considerable time.
Now that there has been an update to the Business Protection software, where the majority of avast 4.8 installations are. As they update to the newer versions avast 4.8 will cease to be supported. Whilst there is no fixed date, it is unlikely to be a long term thing. So now is the time to plan the update if you have win XP, Vista or win7 OS.