Watching 720p and 1080p Movies on a 4K or 5K Monitor I want to buy a 4k or 5k monitor but I'm worried about watching 720p and 1080p movies on it.  I have a 24" 1920x1200 monitor that I watch lots of movies on and just wondering how would I watch movies on a 4k or 5k monitor if it has much more pixels?
Assuming you're talking about playing the movies on your Mac, the Mac will scale the picture up or down to the appropriate pixel size. If you've ever watched a DVD, or DVD-quality, video on your current monitor then you've already seen this in action because the native DVD resolution is only 480 or 576 pixels high. Conversely I can watch 1080p HD video on my iMac with only a 1680x1050 screen; the Mac scales the video down to fit.

Alternatively, you can set the Mac's display resolution to 1920 x 1080 and let the monitor do the scaling. Either way should work.