Your trio sounds completely adorable! Is there pictures of them somewhere?!
I’m not surprised Peanut is the leader in your group. The girls often are. I think if you were to bring in another it would have to be a submissive male or female for it to work I’m guessing. I also think it could definitely change the dynamic so there is that risk. You might end up with a group but some of them pair off within the group OR your oringinal 3 and the newbie not quite intergrated. i.e. in the group but on the outside a bit. There is lots of scenarios I guess, even a new one intergrating well but one of your current rabbits being ostricised somewhat. It could take some work.
Having said all that, I have a friend who adds rabbits into her group with little trouble so…. I think if you’re keen to adopt this rabbit, take him on a trial basis and see how your trio reacts to him. I’d be too nervous to try it but I’d love to hear if it was a success for you!