You'll need to specify which type of accessible room you need at the time of booking. Do you need a 'modified' one with grab bars, raised toilet etc. (which still has a tub)? Or do you need a room with that and a roll in shower? Both are different types of rooms the special needs dep. assign. As both have a different set up, they are layed out with small differences.
Which ever type you need though; simply go ahead and book a room for the 4 of you if that's what you want. As long as a regular room can accomodate 4 (which they can at CBR), you can book the same amount of people into an accessible room. Disney will make arrangements, if need be, to accomodate a sleepingplace for all. How this will be taken care of differs per situation, resort, type of room etc. But rule of thumb; any amount of folks you can book into a category room, you can book the same amount of folks into an accessible room within that same cat at that same resort.