Do tvs upscale videos when connected through a hdmi from a computer? Say I have a 4k tv and my graphics card is capable of outputting 4k as native res and I connect my pc to my tv. I then play a 1080p video on my pc. Will my tv upscale the 1080p video or leave it alone because the pc is outputting 4k from the graphics card even though the video itself is only 1080p?
Yes and no - yes the video will be upscaled, no it will not be done by the TV (for the exact reason you stated; because the TV is only "seeing" a 4K video feed). The computer, however, will upscale the video to 4K (modern graphics cards are capable of doing this very well) if you need the video to fill up the whole display (versus if you watch it in a window and have other applications/windows open on the screen, it won't be scaled up to 4K, it will only occupy however many pixels the window occupies).
If you set the PC's output to 1080p, and the TV is capable of scaling (not all 4K TVs are; refer to the owner's manual) it would then scale up to 4K. If the TV isn't capable of scaling, there are inexpensive adapters like this one: http://seikistore.com/collections/audio-and-accessories/products/su4kc1
that can perform upscaling to 4K from 1080p.