Depending on where the bottleneck is in your computer, the 4k video could just be too much data throughput for the computer to keep up with. If that's the case, a smaller resolution and slower framerate should help. Or if you have software to compress the video (such as handbrake) you should be able to let your computer turn the videos into much smaller more manageable files (it will take a while to run though). I've found the 4k videos look better than 1080, even though I'm looking at them on a 1080 display. Letting something other than the camera downsample provides better results in my experience.