Is cpu or gpu being effected when playing 4k video files on pc.?

You need to post the entire specs of your PC and exactly which HDMI cable you are using, as well as the software you are using to play the videos.

4k is very demanding. You'd need to upgrade your GPU, everything else should be fine. You are seeing a lot of cpu power being used because each player becomes unreponsive due to its lagging which is caused by the GPU's weakness... or something along those lines lol.

upgrade your GPU drivers first before doing anything rash like upgrading your hardware and see if that works. good luck

playing 4k videos takes alot of CPU power, and the fact that u are still on windows 7 is part of the issue.

since you already ruled out the cpu the only two things that could cause lag are

1. the GPU (but video decoding is generally very light on the gpu)

2. the storage (your hard drive could be too slow for the video and thus causing lag)

A 4k video is not a 4k video on a 1080p monitor, it's only 1080p, thus no lag. There should be a gpu monitor as well in the task manager that you could look at to see if it is near 100% while playing the lagging videos. It's only while the computer is lagging (under load) that you should be looking at the task manager. If neither the CPU or GPU is under load, during the lagging, then you have some other problem.

The effect when someone mistakes effect and affect is that readers are affected by their mistake.