So this is ULTRA HD but I've read that there aren't any (or very few) computers or other devices that can take advantage of it. So is there any reason to use it? I'm using VLC to watch a Ultra HD movie I shot today. I can rotate the video (it came out sideways) but it won't play right. Video stops and audio plays...even after I close the program! Don't know if Ultra HD is the problem or what. Uninstalled, reinstalled the program but it didn't help.
Re: 4K Video
Well, make sure you're recording videos in landscape and not portrait, so they won't come out side ways.
I recorded a 4K video from a Note 3, which I transferred to my S4 at the store. I couldn't play the video on my S4, but the VLC player on my PC was able to down convert the 4K/2160p video to 1080p and it played back flawlessly on my PC. It could be your video card as the 4K video averages close to 50 Mbps as oppose to the 1080p video which is closer to 17 Mbps @ 30fps or 28 Mbps at 60fps. So it may be that the video card that it's giving you problems.
Main reason to record in 4K is that years from now, you will likely have a 4K tv and you may want to revisit those memories in the best resolution possible. Your S5 is pretty much your only source for recording 4K video at this point, unless you want to spend a pretty penny on a 4K camcorder. On top of that, a 4K video you record on your S5, will yield better results than the native 1080p recording of the S5. With a program like DVD Architect, for example, you can down convert those 4K videos into much higher quality 1080p videos.
All of this is if you don't have a dedicated 1080p camcorder which records significantly better HD videos than any phone. In that case, you can record the 4K videos on your S5 and save them for a future time when you have the technology to appreciate the quality. Even on a QHD phone you will be able to appreciate the 4K quality more so as well.
That's my plan anyways. I have a Canon camcorder for my 1080p videos, and when I end up buying the Galaxy Note 4, I will be recording a lot of clips in 4K and saving them for future use when I upgrade my displays.