A discussion about OLED screens for FP4 was already seen here, it may be better to continue it down there than in this topic 
Maybe, but very probably not.
I think it won’t come before a year or two at least.
As to whether it will have an OLED or not, I believe so, because all recent phones have one. I doubt FP will still put an LCD on the FP4.
But if you want your voice to be heard by FP and to have official answers, please contactsupport!