Bob, this is actually by design.
Version 4.5 has been modified in the sense that program updates (unlike VPS updates) are distributed slower. Namely, during a period of one week (7 days).
This feature has been implemented to avoid congestion of our servers. Say update 4.1->4.5 will have almost 2 megs. With the current number of avast users we wouldn't be able to supply such a bandwidth in such a short time. Distributing the update evenly over a longer time period will help us mitigate the server load (and provide smoother update to everyone).
Of course, this does not apply to VPS updates that are delivered always on time (but are orders of magnitune smaller).
Hope this helps.
Vlk