well even if your rabbit didn't get her pregnant, it sounds like she may have already been mated by the previous owner's rabbits if she was in a group of 4 - or had he seperated the sexes?
Like honeybunny says - your boy will be fertile 6 weeks AFTER his neuter too, so keep them seperate.
You really need to find a good vet to get this girl spayed this week - or there will probably be babies in 4 weeks time, and she's really too young to cope (and note - she can get pregnant again immediately after giving birth, so keep them seperate still)
A good rabbit vet will use isofluorane gas anaesthetic, which is much lower risk than what they use for cats and dogs, will give post-op painkillers, and will do invisible stitches, as rabbits are notorious for pulling them out (and wearing a collar isn't ideal as rabbits need to eat their squishy poops) - Never starve a rabbit before surgery, and do keep them warm and try to encourage them to eat and drink from a dish afterwards - or it's back to the vets in a day because rabbits MUST keep eating constantly (hay hay and more hay).
You will still need to get your boy neutered even when she's spayed, as she's unlikely to tolerate being mounted constantly.