Snagged a fde federal G rail in Black Friday and have a mk18 upper coming in the mail, was wondering if there were some pictures out there of similar lengths? Tried googling but hadn’t found much

No pics of that specific configuration, but know that with a 10" rail over a 10.3" barrel, most of the muzzle threads (about 0.5") and this the base of any muzzle device used will sit under the rail.

Shouldn’t be any issues though correct?

Depending on the muzzle device, you could potentially have clearance problems (mainly thinking of some of the larger devices). I kinda doubt you'd have enough clearance for a suppressor if you wanted to run one; not sure the internal dimensions of the Mk4, but I think it's fairly tight to the barrel. Otherwise, I'd think it'd probably work fine.

Personally, I'd probably run an 11.5" or get a barrel chopped to something like 10.8" to bring the shoulder even with the end of the rail.