I’m afraid Avast 7 certainly won’t work on Win9x - since Avast 5.0 the system requirement has been Windows 2000 SP4 at least (and Windows XP SP2 since Avast 9.0).

As for Avast 4.x - even though the virus database got reduced, please keep in mind that Avast 4.x is long past its support and is only getting a part of the detections the higher versions are receiving (because its “engine” is not capable of new types of detections). So the detection capabilities of Avast 4.x are significantly lower than current versions of Avast and it really doesn’t provide full protection.