These fish can get to 1 foot, but they hardly ever do when being kept in an aquarium their whole lives. Still, I think 6 ft is the minimum for an adult. I have a 7" blue hippo and it is very active. Swims from one end of the tank to the other all day long. It's hard for me to imagine putting her in to a tank only 4' wide. I think a yellow or kole would work fine in a 4' for life, but definitely not a blue. I'm speaking off of personal, current experience. Not off of things I read on here.
That being said, you've got years before it will reach the size where you'd need to get rid of it (5-6 inches). There are deals all over the place on 125 gallon tanks. I think you should hold out for a 6' tank if you intend on keeping larger fish.