The OP never actually mentions 4K video. For all we know, he's recording at 1080p. I can playback 4K video just fine on a 2009 MacBook Pro - but I wouldn't want to try editing it on that platform (as it would take forever).
Not copying the video files off of the SD card to the local hard drive before attempting to play them seems like the most likely cause for his playback problems. It may also be the playback program he is using. He should make sure he tries QuickTime - some other program may have hijacked his file association for the video files.
While you are correct the format doesn't matter. Some devices with standard video configurations can't handle it. If their system was a base model it's likely to have a 94m 256 video or integrated. If yours can play it fine, you likely have the upgraded video card version. At the time apple offered a 1gb version which can play even 4K fine.