If you have a 4k project and the bandwidth or time to upload, I would upload in 4k at a higher bitrate. YouTube will transcode the media to several resolutions so end users will get what's best for their device. If you plan on keeping videos on YouTube for several years, I just think it makes sense to upload in the max resolution to make it more future proof at least that's what I do.
I use FilmicPro on my iPhone XS Max and use two mirrorless cameras that record at 4k 100mbps as well as a Mavic Air. There is no sense, however, exporting in a bitrate higher than your capture rate. I believe the built-in iOS camera app has a lower bitrate than FilmicPro.
Yes filmic pro is much higher bitrate. but i stil luse the built in ios camera app on my XS because i really like the way the stereo mics sound. Filmic pro and all other 3rd party apps can only use one microphone at a time in Mono because apple never released the stereo mic API for 3rd party apps. But i will see if there is a way with an adapter to use my ZOOM H2n as a usb mic with filmic pro so that way i can use filmic pro and have good audio as well. If it wasnt for filmic pro only doing mono 1 microphone, i would have switched to it already because i really want to be able to lock exposure/WB in filmic and shoot in LOG