Watching 720p and 1080p Movies on a 4K or 5K Monitor
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.