MacBook Pro retina display resolution 4k?

How can I display at max resolution on this Mac?

currently I have it set the the default 1680*1050 (looks like) but under displays is says 2800*1800 so this I understand is the retina scaling.

However on youtube I get this:

1680 x 945 *2

So what resolution am I running at now?

If I change to (more space) (1920*1200) I get this:

1920 x 1080 *2

NOW if I watch 4K footage in VLC I get this:

Similarly if I take a screen shot it will take a 4K image at 144ppi

As you will also notice all screenshots are also 3840*2400 or 3360*2100

Am I to believe here that VLC is playing at 3840*2160 on a 2800*1800 monitor ??

I am used to pixels and photos and 1080p but retina is beyond me at the moment to explain.

Currently I'm trying to get away from. my old 17" 1920*1200 and hoping this 1680*1050 doesn't look too cartoony and massive atm, as 1920*1200 on a 15" screen is a little eye straining.