To me, the most important quality in the beginner bass is comfortability. If your instrument is not comfortable for you to play, you will not practice as much or as easily on it, and if you push, you might cause an injury to yourself.
Finding good, comfortable 6-string bass is way more difficult than 4- or even 5-string, because there is less basses to choose from and because seemingly minor details like string spacing, fretboard radius, neck profile differences and such will suddenly become a HUGE deal compared to 4-string basses.
On the other hand, finding a comfortable 4-string is much easier. 5-string, not as much, but still easier.
Finding good, comfortable 6-string bass is way more difficult than 4- or even 5-string, because there is less basses to choose from and because seemingly minor details like string spacing, fretboard radius, neck profile differences and such will suddenly become a HUGE deal compared to 4-string basses.
On the other hand, finding a comfortable 4-string is much easier. 5-string, not as much, but still easier.