What makes the most sense os besides android
Welcome to the forum.
You’re giving us very little to work with. Could you elaborate on your question and which problem you’re trying to solve? What I gather so far is that you want to use another OS that is not Android on your FP4?
Adding to what @mde said - What operating system to replace your stock ROM with is always a question of your usecase and motivation. No system is perfect for everyone. Without knowing a bit more we can’t really give any useful recommendations. Here are some questions you could/should answer:
Why do you want to use a different OS in the first place? Do you rely on certain proprietary features or applications? Do you simply want a de-googled device? Why a different OS but not an android custom ROM? How much time and effort are you willing to invest into manually setting up your device and keeping it running?
If you want do de-google specifically - e/OS/ is an android version / Distro that comes entirely free of google stuff, and is Fairphone’s official partner project (you can even purchase a FP with that pre-installed from the Murena store). It replaces all Google services and apps, using MicroG for the services specifically.
It attempts to be as compatible with conventional apps as possible, with only a few caveats like in-app purchases not being supported, and paid apps from the appstore itself, if you don’t create and use a Google mail account for that. That would be the only “big” issue that comes to mind.
In return though, you get a system without Google, and with advanced privacy settings integrated at system level. Including the blocking of tracking and most if not all ads, in any and all apps you download. You even get to pick and choose or automatically randomize your GPS location, for all the apps that insist on you sharing it with them. That part is pretty epic, too.
I’ve been using e/OS/ for more than a year now and the open source appstore F-Droid. In combination, that’s pretty nice. If that floats your boat, feel free to ask more specific questions about that and I’ll answer.
There’s also DivestOS.