40" 4k monitors are running around $250. I have a 39", a 42", and a 55" ($340) that I haven't hooked up yet (I do watch movies on it). The 39" now looks small to me. I'm a programmer so screen real estate is a big plus.
I am working on my Seiki 39" 4k right now (I also have 42" and 55" 4k monitors). The text is very sharp and easy to read. When you buy a 4k tv you need to go in and change the settings so that it works as a monitor. My vision sucks, these are the best monitors I have ever used.
Your choice would be basically any decent laptop from any brand be it Dell be it Hewlett Packard for something like $500 like you listed, it's on the low end for laptops, but you will really get some machine with integrated graphics for PC for example.