Hello Everyone,
Since I've faced a little problem with my 4K TV recently and found the solution, I thought it would be nice to post it here in case it might help people facing the same problem.
On my first boot, the TV showed the boot screen without a problem (not cropped) and then showed "Invalid Signal" once it has loaded Recalbox. I changed the settings in the recalbox.conf from CEA 4 (default) to CEA 16. That solved the problem regarding the "Invalid Signal" however I faced another problem once Recalbox had booted.
The new problem, was that my screen was cropped (both sides but more on the right side 1/3 of the screen off)
I've tried to plug the box in 3 HDMI port and it didn't work.
I finally tried the 4th HDMI port which is separated from the 3 others (on the back instead of side). The TV recognised the system on boot and did setup the screen for it. It worked perfectly. I did swap my HDMI cable back to the side HDMI (where it used to be a problem) and the TV apparently memorised the setup. The TV is now properly showing my Recalbox without any crop problem.
TV with internal chipset are practical, but you have to tame them
TV model LG C9
I hope this will be useful to others ^^