Uploading 4K horizontal videos using the web version.

I am trying to upload very high quality video's of around 10-15 seconds. I uploaded one but sometimes it's high quality for myself, but if i watch it on another account it's low quality.

Is my video going to be 4K for everyone or not? And is this fixable or do i need to stick with 1080p?

It's a .mov file btw. MP4 is a premium function on the video-editor i am using.