I have 4k source material from three cameras. I know the output will be 1080p for Youtube. When working in FCPX is there any difference in final product quality between setting the multi-cam clips and project to 1080p for the editing session OR setting them to 4k and as a last step rendering the final product to 1080p?

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 courses—maybe a true FCPX guru can offer a more definitive reply..

Course author here. It turns out that you can get slightly sharper output if you edit 4K material in a 4K timeline and then export 1080p from there. However, if you scale the video at all (punching in) then the sharpness advantage disappears. The difference is fairly minor, but you may be able to see it.

Thanks! This is what I determined from minimal testing but I figured I would ask here as well.