I haven't cooked 4 chicken halves. But I think they will fit.
I just used my mini and got it easily up to 300-350 with no problems. At one point I had to close the bottom vent a little it was creeping up to 400
It depends on how your mini is set up for how hot it will get. I just use the steamer pan with the bottom cut out and can get over 400 but i do my chix @ 350 ish on average.
I'm not a fan of two rack cooking of chicken because the bottom will finish first and i'd have to lift the top to pull the bottom. If i were doing 4 halves i have this cookie rack that i'd put on my mini top rack and do 2 on each.
I can easily fit 2 spatch-cocked chickens in the mini using both grates and maintaining high temps is no problem.
But if I were doing 2 chickens, it's a lot easier for me to just do them on the UDS.