With my C3 when I give the TV a 720p 60hz signal with the Boost Mode (for input lag in game mode) There is a horizontal line of tearing near the top of the screen. From my testing it stops tearing when I turn the input lag setting to standard or use a different resolution like 1080p. I tested this on a Nintendo Switch by changing its output resolution setting.

I swapped HDMI cables known to be good and swapped HDMI ports. I also tested with my SNES classic which is default 720p 60hz and it had the same line of tearing at the top of the screen. Seems like it’s an issue with the C3 with Boost mode. Several months ago when I first encountered this issue I sent it to LG just in case they eventually fix the issue. I’ve had several LG C3 firmware updates since then but every time I test the issue it still exists. It’s not a huge deal to me, just a weird quirk of my C3.

Have you seen this on your C4?