Maxing out the settings in BF4 ( MSAA 4x ), I was happily the game between 60 - 70 fps. VRAM was using up 1700 MB ( which is a no brainer thanks to MSAA 4x ). Then I come across this neat little feature called 'Resolution Scaling'. Now obviously, seeing something new, my curiosity triggered forcing me to increase the scaling to 200%. Gameplay performance: 20 FPS. Wow! VRAM 3020 MB. 
Card: R9 280X

So theoretically, I was gaming at 4K resolutions?

It basically is Supersampling AA.  It renders the image at a higher resolution than what is being displayed, then shrinks the image down to the displayed resolution.  The result is a sharper image (usually better than MSAA), but takes a lot of extra horsepower to achieve.  SSAA gives great results but is the most system taxing form of AA there is.
 
But no, you were not gaming at UHD resolution.  1080p = 2 million pixels, so 200% would be 4 million pixels (not sure of the actual resolution but is a little more than 2560x1440 which has 3.7 million pixels); where UHD (3840×2160) has more than 8 million pixels.  So you would need to be gaming at 1080p with 400% scaling to be rendering at UHD.
 

amtek
Maybe..
 
Playing at 100% scaling @1680x1050 doesn't look all that impressive to me, it feels lacking like its not really 1680x1050, too many jagged lines even with 4xAA.   But once It's scaled up to 115 or 120% it looks like the game should at that resolution, nice and crisp with no jagged lines. 

100% scaling means that it is just running at the current resolution you have it set as, which means that you kind of have the option disabled (100% of 1680x1050 is still 1680x1050).  That means you won't see any benefits from the scaling option since you are keeping it at the same resolution.  You need to set it above 100% to see any benefit.

Resolution and pixel count are the same thing.  Resolution is also measured in Megapixels.  You have to understand that doubling the height AND doubling the width is actually quadrupling the resolution.  UHD is 4 times the resolution of 1080p, or 400%.  You can stack 2 1080p screens side by side, and 2 1080p screens above them to get UHD.  You have to do 200% of 1920x1080 combined since that is the resolution.  You can't take 200% of 1920 and also 200% of 1080, because that adds up to 400% again.
 

OV3RCLK4
If it goes by pixel count then 2880x1620 should be more demanding than 1920x1080 @ 200% but its actually the opposite.

Do you have any comparisons that show 2880x1620 resolution and 1920x1080 @200%?  Also remember that when you are using the scaling option, that you are doing extra work for the GPU.  When you are scaling, the GPU has to render the image at the blown up size, then out of that image, make another smaller image.  So it has to go through more processes than just having a higher resolution.

Is there any benefit to running a higher internal resolution than your monitor can display? Will textures at a distance look better when you snipe or anything like that?

I used to use the down-sampling technique to force my older games to run at crazy resolutions.
Cool that it's becoming an option in games without having to do this. 
http://www.neogaf.com/forum/showthread.php?t=509076