For YouTube, and web video generally, 1080p hi-def is a legit format that will be fine for quite a while. The default format for Facebook is still 720p, I think..
The advantage of 4K gear is it's future-proof. The industry is heading that direction. Some folks I know are getting into 8K
.. with the idea, though, their work will go on 98 inch TV screens. It's ALL about resolution, and the intended audience.
As a performer, if you want to get your material on TV or added to a documentary, you'll want higher quality than 720.. or even 1080.. Having said that, a GREAT performance will make a documentary cut even if the producers have to jazz up the resolution somehow.
I'd say the most important thing is to get better at 1) performing, 2) video, 3) marketing, etc., so do what you can afford NOW and keep going!!
After all, a guy I know with a RED camera still busts out his iPhone to take video once in a while...