No. 3840 x 1920 = 7372800 3839 x 1920 = 7370880 3840 x 1919 = 7368960 If you remove 1 pixel from vertical you remove 3840 pixel, but when you remove 1 pixel from horizontal you only remove 1920 pixel.
4K ist just the new selling point in TV as it was 3D before. Most people don't care a lot in picture Quality as they still don't complain about TV Program in < Full HD. And most people as well are sitting so far away for their TV they won't notice a difference to 4k, and I guess FullHD neither.
@Andrew: Some tip for Your 5k iMac. If you hold Option while clicking on "Scaled" then it'll let you choose 5120x2880. So You are able to use 1:1 Pixel.
The new retina iMac is inbetween. It's even faster in some tasks then some of the Mac pros and gives u an amazing screen. http://barefeats.com/imac5k.html