I always shoot in 4k (or at least 2.7k) and then output my footage at 1080p.
The reason to do this is that the 4k resolution lets you crop, pan and smooth your final video.
If you look at your video about half way through there is a white bird standing on a fence. But it is very small. Now if you zoom in 2x, you have 1080p footage of the bird with the bird largely filling the frame (which I think would be quite a bit more interesting.)
I am not convinced that 4k makes much difference when viewed on youtube (and it requires a faster connection). However using 4k footage is very helpful in creating a better overall video.
Unfortunately, editing 4k really needs a fairly powerful computer with a decent video card.