her belly is very round and firm It will be even with water retention, though to be honest re: her scan if you could actually see 4 or more then I've always doubled that (I have always been proved right and just use the scan to give me peace of mind, never a true indication) so, be prepared it may well all be pups, you never know............
You could try to palp and listen for heartbeats and give yourself an idea too. In agreement with everyone though, size of the bitch in whelp is no indication of litter size at all, I've seen bitches who hardly look 6 weeks and they have had large litters with pups hidden under rib cage etc, and large, 'I can hardly walk' bitches who have had just the average size litter.
I've even had my niece so huge (she was massive) and so petite have the midwives saying she was about to have a large child and she gave birth to a wee 6lb babe.

Just be prepared for anything.