I have the catit cabrio just now for 3 but I wouldn’t fit a fourth bunny in there too. I have seen a ferplast atlas 40 (without wheels I think) on gumtree available fairly locally so was considering that one but hadn’t considered whether it will fit in the car! Small dog crate might be an option, presumably you drape a blanket or something over it so the rabbits have cover especially in the vets waiting room?