Do it all together. Do it the same way as you would for bonding two bunnies, just treat each pair as though it was one bunny. There's a lot more scuffling to sort out, and it's MUCH scarier, but separating them from their bonded partner will just cause additional stress.
Our four are the same as yours. We bonded Chips and Cassidy to the two lop sisters. Chips mostly cowered in the corner while Cassidy went bonkers and tried to kill the girls. They were, fortunately, very submissive to her. Neutral territory is going to be an absolute godsend if you can find it.
The thing we found that worked best was putting all four in the boot together, with their carry cases open, then driving them round to Andi's mum's where we had a run in the garden. Dumped them all in the run for a couple of hours and then drove them back in the boot.
In the car, all four would huddle together in one carry case. The run was completely neutral, so there'd be some fighting but mostly it was a standoff. When we got home we'd try leaving them together, but the fighting would kick off immediately. Eventually, we got to the point where they were grooming each other in the run, snuggling in the boot, but fighting in the house and we just had to bite the bullet and let them have the final power struggle in the house.
Even now Cassidy still has to have the odd nip at the two girls. Nothing serious, just a bit of fur pulling, but all four will snuggle up together quite happily. I think she's just particularly aggressive and needs to give them constant reminders of how much she should be feared.