Mentioned earlier but it’s my main thought:
string spacing.
I own two 4-strings and one 5-string (one of the first BTBs). I bought the BTB 5 thinking that similar string spacing (to my 4s) would help me learn the 5. That was correct, but until I joined a band that asked for a 5, I barely used my 5. For almost 18 months now I’ve used the 5 exclusively in this newest band, while using my 4s for other gigs and church. The “
exclusiveness” was the key to me getting used to (and proficient on) the 5.
I’m getting ready to pull the trigger on a backup 5 (Sire M2 – 18mm spacing) and while shopping in person I tried a few 5ers that had narrow(-er) string spacing than my BTB (19mm I
think). WOW even for the songs/lines I play in the new band on the 5, I was totally lost – the string spacing was SO different. None of my “moves” were working, and I just couldn’t figure out which string I was playing.
The initial use of a 5 vs. a 4 is tough IMO due to the fundamental nature of the string positioning. I compare it to trying to drive a new car with a steering wheel in the middle of the dash instead of the left driver’s position, or playing drums with the snare outside my left or right leg instead of between my legs…it’s truly a fundamentally different positioning, which takes a good amount of experience to get used to.
Like some have said I now find it tricky playing the 5 then going back to the 4. I still prefer playing my 4s, but it’s sure nice (esp. in church) to have a low Eb and D.

And changing keys (which all my bands do a LOT) is much easier on the 5.