It really doesn't matter if you get a 4 or a 5. The only thing that matters is your personal preference and how hard you are willing to work at becoming a good bass player. If you know your're going to want a 5 string later then cut to the chase and get it now. Victor Wooten is about as advanced as they come and most of the time you see him with a 4. Then there's John Patitucci with his 6. Just make sure whatever you get is made out of a good-sounding wood, has durable hardware, and has been set up properly and you'll be fine.