I started with 4, transitioned to 5, and then transitioned to 6.
When you first start the transition, your overall playing ability can be significantly degraded. It took me a couple of weeks to get comfortable approaching the instrument as a 4 with an extra string. I had to be very purposeful about how I used the extra string. Within a few months I was reasonably comfortable, but still could get lost/confuse on the fingerboard from time to time.
The extra strings tend to have certain benefits in the way you navigate the fretboard. More strings allow you to cover a wider range of notes with less shifts. So over time the logic you use changes. Rather than shifting up and down the neck, you tend to play across the strings more. Ideally the finger patterns you use work on a subconscious level and are essentially automatic, I.E. you don't have to think about them.
If you play the 5-string exclusively and practice enough, it will eventually become your native instrument. You will think as a 5-string player, rather than as a 4-string player with an extra string. I never made it this far on the 5, and it took about a decade for me to get to this point on the 6. Staying there remains practice.
I have one 6-string in my collection and a variety of 4s and 5s. The 6 is my #1. I find that I can pickup a 4 or 5 and play very well. But if the music is technical, I have to stop more and figure out how to play the line without all 6 strings. I find that if you play 4 or 5 too much that it degrades my proficiency on the 6, because it starts to modify my subconscious mental processes.
If you want the transition to go as smoothly and quickly as possible, pay the 5 exclusively until you get really proficient. After that, make a decision on which instrument you think is most import.