Windows 7 displays horrible on 50 inch 4K TV

the link above discusses upgrading to a new radeon r7 card which will get you native 4k resolution. If you have a dell 8500 you probably have a radeon 7870 card right now, which is less than 4k. Before you consider upgrading I suggest you first set it up perfectly and decide how you like it, because 2560x1440 is not too shabby either.

Your 4k monitor has a native resolution of 3840x2160, which is 16:9. Therefore you want your graphics card resolution to be 16:9. The radeon 7870 card has a max resolution of 2560x1600, but that's 16:10, so you'll want to use 2560x1440 instead. You will also want to use its displayport output as opposed to hdmi, which gives you double the refresh rate according to AMD.
Radeonâ„¢ HD 7800 Series Graphics Cards | AMD

Once you are set to 2560x1440 its possible your screen will still not look right (true for my set up, though my tv is not 4x) and if so you have two ways to fix it. The first is by using the TV's remote and go into display settings (note this is not accessible from windows, this is using your tv's built in menu system) and try different display options there. On my tv there is wide, normal, etc. I can't remember which one works best but I had to play with them to figure it out. The second way to get your picture right is to use the radeon graphics control panel (or whatever its called). In there will be some "scaling" settings you can play with. On my system (which is intel graphics) those are called things like adjust aspect ratio, scale to full screen, customize image). Once you set these up your picture will be great. I had to customize my scaling because otherwise the icons were off the screen but once I set it up using the graphics control panel, now its perfect.