Render files can always be deleted. FCPX Will automatically regenerate them when needed. You will have to wait while it does it however.
A partial, and maybe better solution is to delete just the unused render files left over from previous work. FCPX has the command under the file menu to do this when the library is selected.
you can also tell FCPX to store the render files on a different disc, and easily delete them from that disk.
One of the best solutions however is buying more discs. They are always getting cheaper.8 TB drives are available and quite cheap. Check Best Buy for deals on Westetn Digital drives.
I do a lot of video work, some involving drone footage. I generally have 30 TB mounted, and another 20 or so off line available if needed.
There is no such thing as too much disc space for 4K video.