Family of 5
We have 3 children and cruise at least once per year. There are various answers to your question. Initially, as you suggest, if the max per cabin is 4, then it does not matter if you have 3 infants. But we can get this done anyway.
Older Carnival ships do allow for 5 in a cabin. We did this on the Triumph and had a good time. We did book a Cat 12 suite which gave us plenty of room. When you book a suite only the first 2 prices are higher, the 3rd, 4th, and 5th fares are usually the same ($199-$399) regardless of what type of cabin you are in. Thus when we went on the Triumph, we actually got a bigger, nicer cabin for less than the cost of 2 separate cabins because the 3rd and 4th passengers were cheaper. Unfortunately, with the exception of the family suites on other lines, only Carnival will book 5 in a regular room and this is not on their newer ships.
You mentioned travelling with other family members. When the kids are young, book one with grandma and 2 in your room. The little one stays with you anyway. Caution though....make sure there is enough room for the crib in the room. We did mini-suites on Princess this way and had a great time. My youngest was in a porta crib between the foldout couch and our bed and things were great. Unfortunately, now that my kids are 9/6/4 this is no longer an option.
Adjoining cabins are an obvious choice. But not necessary with very young kids. There seem to be fewer adjoining cabins on the newer ships. Biggest drawback is 4 "full fares" and only 1 "kids fare". We have done this on Celebrity Constellation and things were fine. Obviously offers Mom/Dad some private time.
We did one cruise on Carnival with another family of 5 last year. The ship was a 4 to a room only. We booked 2 Cat 11 mini-suites and 1 small inside cabin. The extra cabin was there but for the most part never used. I even took a mattress out of the room and hid it under our bed for nighttime use. On the last morning, I put it back. Tipped the cabin steward regular tips for doing almost nothing.
Finally next month, we are going BIG. As in Big Cabin and Big Bill, We booked a Family Suite on Royal Caribbean. Sleeps up to 8. There are 2 separate bedrooms and sitting room with Big Balcony. We are very excited. Nice thing about RCCL is, in spite of this being a huge cabin for 8, they still only charge for 2 "full fares" and up to 6 "additional passenger fares". Similar family suite on Princess costs 4 full fares and up to 4 additional passengers.
Family of 5 is a difficult thing when it comes to Cruise ships and for that matter hotel rooms. You won't have that problem for a few years, but in time you will always stay at Embassy Suites because of the roll out couch. There are ways around the 4 person max, you just have to look.
