The new screens are concave.
For most combinations of screen size and viewing distance, 4k televisions won't actually improve resolution - standard HDTV sets already give you all the resolution your eyes can handle unless you have a really big screen or sit really close. And if you don't have some genuine 4k source, be aware that "interpolated" 4k pictures aren't really 4k. HOWEVER, the 4k sets are supposed to have a higher dynamic range - whites are brighter, blacks are blacker, etc. If they're adjusted properly (most store displays are NOT adjusted properly) you can get really black blacks, without obscuring detail of "almost black" parts of the picture.
As a rule of thumb, the middle of the product line is the sweet spot where better panels begin to appear, but the cost doesn't go up insanely. If you have some other stuff you want to plug into the TV - video game, VCR, computer, etc., be aware that the manufacturers are cutting back on the number of connections in order to reduce manufacturing cost. So check out the jacks, too.