So, I originally wanted to reply with...why use 5" paint brush when a 4" works just as well. That thought morphed into the following, after a bit of thought. My personal opinion is in favor of setting up a 4string for BEAD tuning. Although, it would require a pretty specific setup, making the conversion back to EADG a bit of a task. So, in your situation of only being able to own 1 bass...it may be worth looking for a 5er if that's what you want. To address your developmental concerns...as long as you find a 5er that's comfortable for you, I wouldn't worry to much about it. In some cases it can enhance your development as you'll be able to extend the range of the scales and such into a lower register. So, I suppose bottom line, is, if you want to play a 5er, go for it, but definitely try out a few before you buy/trade for one. You seem at a stage where, it's more important to find the right instrument for you, than the number of strings.