So, I don’t have a quad, but I do have a failed one (now two pairs LOL). My experience was I had a M-F pair, then I tried to add a third (F) for a trio, and the girls just weren’t getting a long. I decided to pause and do more pre-bonding/ reassess. Then I fell in love with this other bun, so I decided I would try for a quad, and if that didn’t work out I would bond him to my single so I wouldn’t have a solo bun. I did a few speed dates with all of them and they went OK. I tried for 24/7 bonding with all 4 when I moved to a new house, and my males just were not getting along. They actually started out really well, with some cuddling and even grooming, but then once my older bun settled in a bit he started trying to mount like crazy and it caused a lot of fighting. He is also 12 (almost 13 now!) so I decided it wasn’t worth the stress or injury risk. So I ended up with 2 happy pairs. In all of that, my original pair never split up or fought, but my male was trying to mount her pretty relentlessly so I actually had to separate them a bit to let her catch a break and keep him from having a heart attack. I will say my original pair seemed very relieved that it was back to just the two of them when all was said and done.
So, I think the fact that your littles and your current pair seem pretty mellow is a positive sign for a possible quad! I also think it’s a good sign that your mini rex pair hasn’t really reacted to the new bunnies at all. Did you see any referred aggression? This is when a bun is annoyed at another bun, but lashes out at the nearest bun or human instead. There is always the risk that trying for a quad can disrupt the current pair, so if you really want to make sure they stay solid, the safest bet is to just have two pairs.
That said, I think you could start by finishing up the young bunnies’ bond, just to make housing etc. easier. At four weeks post castration, their hormones are probably mostly settled, especially if you aren’t seeing a lot of marking etc. While it’s true that many bonders will just work with the group all together, it sounds like the young bunnies are practically bonded, so finishing that bond might make things easier for you (with the contingency that it might then be upset again during group bonding). It can also be overwhelming to work with all four, so sometimes bonders will try out the different combos to get a sense of which pairings might be the most trouble, and then they focus on that pair the most.
I think it might be helpful to just try a session or two with the pair and one of the new bunnies or both of them if you have a helper with you. You can gauge how your rex’s behave and then go from there.
I do think housing both pairs side by side once you start the process makes sense, you’ll just have to be prepared to separate if they start squabbling with each other. You can also add a visual barrier that you remove gradually if they get overwhelmed with the other bunnies at first.
I think I got all your questions but let me know if I missed something!
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.