My 5,1 runs like a brand new computer, especially since the update that got rid of the blinking screen. No plans to get a new computer, but if I did, I would get another Pro.
A 2009 MBP still does e-mail, facebook, and movie watching just fine. If you need more than that odds are you wouldn't post such a nebulous question.
Haha, I've tried 4K. I have never seen anything laggier.
Not all that many movies in 4K though, are there? Let alone screens capable of displaying a native 4K video.
The Apple Cinema Display is. Not all of the details though. But it sure is better than 1080p.