I'd say it makes no difference if you're starting from scratch, both 4s and 5s will feel equally alien to you. Fair enough it's harder to pick out a single string with the bow on a 5, but once you're used to it the closeness makes string crossings smoother, faster and easier. 5s are common in orchestras, especially in Germany where people play 5s almost exclusively. Oh and will always find a use for the low notes. For the type of work I do, I wouldn't get booked if I didn't have a fiver or an extension.