Hello Momof3farmboys,
Welcome to the Apple Support Communities! We understand that the 4K videos you have recorded on your iPhone are now pausing during playback. This issue only began after you upgraded the iMac to macOS Big Sur.
To be sure that we've covered all the basics, begin by checking for any updates to macOS Big Sur: How to upgrade to macOS Big Sur.
Once you're certain that the iMac is running the latest version of macOS Big Sur, check and see if the video is stored locally on the iMac, or if it's actually stored in iCloud Photos and streaming to the Mac. If it's in iCloud Photos try downloading it to the iMac and see if there is any change in the playback. If there is no change in Photos with the file stored locally on the iMac continue with the following troubleshooting to try to isolate the issue further:
1 - Quit all apps. If an app will not quit, follow these steps to force it to quit: How to force an app to quit on your Mac.
2 - Restart your Mac: Go to the Apple menu in the top left corner of your screen > Restart.
3 - Hold down the shift key on the left side of the keyboard as you click on the Photos app to launch it. This allows the app to bypass the last saved state for the app. When the app begins bouncing in the dock, you can release the shift key. Once the app is up and running, see if there is any change with the contact information.
4 - If you're seeing the same results, restart in safe mode and test again: How to use safe mode on your Mac.
5 - If still occurring in safe mode you’ll need to create a new user, sign into that new user with your information, and test it out again. This article will show you how: Set up users, guests, and groups on Mac.
Let us know the results.
All the best.
