After a week of swapping the two pairs between hitches to get them used to each other, we introduced all four to each other in the kitchen (neutral turf, slippery floor, perfect for breaking up fights).
It went far, far better than we expected. The two boys got on well with their sister, and their mother seemed wary but dominant. After a few hours of this, they were introduced to their new hutch. There was a little chasing and grumpiness from Chi (the mother) but we were quick to intervene and break any scuffles up before they got out of hand.
In the end, we had a pile of four panting rabbits snuggled up together and watching as I made some final adjustments to their hutch. They have been grooming, cuddling and being rather sociable and adorable.
This morning, I discovered that Chi appears to have been bitten on the ear. We cleaned the wound (which seems neat and not torn) and set her back with the others. She seems to be a force for chaos at times, and will chase/nudge the other three to show who is boss. I can only assume that she irritated one of them enough for them to return fire and give her a bite. She seems happy enough now, grooming herself and eating. Hopefully this is a one off, but we are keeping a close eye on them all the same.