My granddad ran trials with setters back in the 40's, 50's, and early 60's. He always told me "I don't want my young dogs sticking their nose in the older dogs ass". Which is why he always hunted his younger dogs separate. He also always said dogs don't train dogs, handlers do.
It ain't rocket science with 4 dogs, this is how I would do it.
Where does your dog go when it's sick, injured, skunked or you only want to put one dog out hunting? How do you contain an injured tog and drive at the same time? What if the dogs gets skunked? Your dogs never get diarrhea on trips? No thanks. I always have a kennel in the back of my truck. Blankets and fans too....depending. Better to have and not need it, than need it and have them sitting on your bench seat next to you. There are some things I don't want to sit in or smell when I'm driving.