IMO eventually you'll probably want a 5, esp if you do Anthony Jackson stuff like Chaka Khan music, but if your setlist can be accommodated with a 4 in drop tunings, well, there's a lot of good reasons to run with that. Relearning the dropped D string will be a thing but a small thing, and it's worth it for the amount of music out there that's specifically in drop D, so hey. Also, the 5 has its pluses, but only if you're going to use it as a 5-string bass rather than a 4-string with a few extensions.
In the cover band which I briefly played, I brought my 5 mainly because I'm already comfortable with it, but were I going 100% with the lines in the songs we played, the lowest note I needed (for 90s + modern R&B style) was an Eb. So hey.
I like the idea of going 4 with occasional drops enough that I'm considering getting a Hipshot for my Washburn Taurus. And it's not redundant with me having a 5: there are plenty of songs where drop D is way easier than using a 5, and occasionally, songs are only possible with a drop D 4 (Tool: Sober is one I ran into a while back, with pick-strummed open D's and A's on all strings).