Hi,
I specifically made the transition from my FP2 to a FP4 and it went with ease (same manufacturer but preferred to use /e/ OS from then). It has no influence in warranty coverage if self-flashed or as Murena purchase.
The new experience for me was just as interesting and intuitive as with my first FP2.
In case you are keeping two SIM cards atm. you will have to “replace” at least one with a modern eSIM
which FP4 will handle. It’s rather a question if your actual provider offer eSIMs as not all do to date.
See here a list which providers by now have been tested with the FP4.
Getting an eSIM to work (if compatible) to me doesn’t look like a challenge and I think everyone can get it accomplished.
The idea of Type-C plug and charging was to (in best case waste reduction) have only one max. 2 charger/cable (per household/individual) for all devices keeping a Type-C plug. Many notebooks stay stationary powered until being traveled with, so the second for true mobile handsets.
Yes there is a difference in charge speed if the cable or charger cannot deliver, hence negotiate high speed charging with the attached device.
But as real scenario, let’s say you keep a portable computer, notebook, tabled etc. in your household with USB-C charger and cable - then use it. It’s just that simple meanwhile.
With quick charge you may full charge the FP4 battery within 1 hour. Without quick charge it may take 2-3 hours.
Using my FP4 every day, I cannot complaint in any way© (pun not intended 
). Somehow I have the impression reading through this forum with stock Android there are still some software troubles I don’t experience with /e/ OS. But this doesn’t mean a thing as it was similar with my FP2 on Android.
Updates are coming in regular terms and I never had one that broke something but rather fixed or advanced things.
Still there’s not the FP4 but two models with different memory and internal storage capacities.
So this also makes a price difference you may want to consider in your decision. Atm. they are treated both equally by the manufacturer, no one can tell if some day one version will be preferred over the other depending on updates or so.
Who knows, maybe soon you’ll decide for the upcoming FP5 model.
Good luck with your decision.