Surprisingly the Windows 10 Photo viewer can clip out sections from video and save them as new files.
* Usually * problems with playback are due to the computer barely coping, especially with 4K
Sometimes a different player will work better. I find Windows Media Player is good as anything on a borderline laptop, however you might need updated video codecs.
A further pitfall is trying to play back without first downloading the file to computer.
Even if you take out the card and have a built in card reader the transfer speed might not be fast enough to play from it, but a USB3 card reader in a (blue) USB3 socket might be OK
The phone cached recording is usually only 720p and a much lower bitrate too.