4 feet is pretty narrow, so it may end up feeling cramped while you're moving around in there, but it's really going to depend on how you use the space inside. For example - are you going to do internal nest boxes or nest boxes that extend outside of a wall? Putting your roosts and/or nest boxes on the short sides vs. the long will keep you from feeling "squished" between them trying to move around in the interior - or even putting the nest boxes under the roost and using the top of the nest box as a poop board. Are you planning to feed/water inside the coop or out?
Are you certain you will not want to expand the flock? I always encourage to build for the largest flock you might ever have - this avoids having to remodel/expand or build more coops if/when you do expand the flock and, in the meantime, just means your birds have all that much more room to live in.