Are you saying "playing solo" i.e. unaccompanied, or "soloing" i.e., playing melodic lines?
For the former, because of the low range of the bass instrument, root-third-seventh for comping will get you across the road and back, so an extra string may not be necessary; extended voicings may obviate the need for a higher string, as a ninth will sound like mud voiced next to the rood (for obvious reasons). For the latter, better ideas and better ears will give you better solos, not a bigger instrument, although extra strings can be handy for extended arpeggios or whatever you may hear.
So, go with more strings if it lends itself to what you're actually trying to accomplish as opposed to what you *feel* you're trying to accomplish. That just means try using 4 first and see how far you can get with what you have.
For the former, because of the low range of the bass instrument, root-third-seventh for comping will get you across the road and back, so an extra string may not be necessary; extended voicings may obviate the need for a higher string, as a ninth will sound like mud voiced next to the rood (for obvious reasons). For the latter, better ideas and better ears will give you better solos, not a bigger instrument, although extra strings can be handy for extended arpeggios or whatever you may hear.
So, go with more strings if it lends itself to what you're actually trying to accomplish as opposed to what you *feel* you're trying to accomplish. That just means try using 4 first and see how far you can get with what you have.