Hi rc415 -
I can't say for FCPX, but with the video program I use for my videos here, after some tests I determined that if the final output is 1080 it doesn't make a difference if I run the project at 1080 or 4k, but (of course) it does if I output at full (4k) resolution. However, I don't know if that would be the case in FCPX, or if working at 4k (or a FCPX-compatible format of equivalent resolution) throughout would result in higher-quality video even in a final 1080 output. (At least one source I found seemed to suggest that there would be increased 1080 quality even if if the 4k video was downscaled to a 1080 timeline to work with in FCPX, but I can't vouch for the source). I don't have FCPX installed here to test, so I'll have to defer to the FCPX specialists on that..
I've linked the thread to one of our FCPX coursesâ€”maybe a true FCPX guru can offer a more definitive reply..