Ah ight, so I did have the 60 and 120 switched. I imagine display port would be the way to go then. I'm not sure how difficult it is to find a 4k monitor with a displayport input, I currently use a display port -> hdmi adapter on my own.
Apple is rumored to be releasing a retina macbook air sometime this year if you'd be interested in one of those, their Macbook Pro Retina is super nice, but super expensive. I don't really have any suggestions for monitors tbh, but if you're coding, I imagine a 4k monitor would be fine now. I'd be much more wary if you plan on watching 4k movies/shows/attempting to game.
Regarding the streaming, Netflix may cost extra, but Amazon Prime includes free 4k streaming. The 4k streaming requires round a 25mb/s connection, but there are compression innovations in the works that they hope will nearly halve the size of 4k. With 4k's current size, movies can't even fit on one single-layered BluRay dvd (which is what most players use.)