Welcome to the forum 
For those who wish to acquire a FP4 and use it with IodéOS I think the safest route is to buy the phone with the IodéOS pre-installed. That won’t help you, but it might help others reading this.
You should certainly prepare your operations carefully and take expert advice (which I personally cannot give you). The iode.tech site is probably the best place to go, though there are knowledgeable contributors on this forum, I’m thinking of @LibrePhoner , @brotmaa for example.
Make sure you read this, cited here:
… and all other topics on this forum concerning IodéOS.
Lastly, be aware that using custom ROMs usually implies some compromises, and certainly care and patience. Take your time!
Good luck! 