It's really better to think of the resolution in terms of @2x, @3x, assuming the screen size remains the same.
All iPads have a base resolution or 1024 x 768 in a 4:3 aspect ratio configuration (i.e. 1024 / 768 = 4 / 3). Retina devices are @2x, meaning 2048 x 1536. To contrast, "Full HD" is 1920 x 1080 (ratio 16:9) and "2k" is 2560 x 1440 (also 16:9). A @2x iPad could thus display full HD comfortably, but not 2k.
If we consider @3x (which is very likely given
https://www.macrumors.com/2014/10/08/ipad-retina-hd-display/), we'd be talking 3072 x 2304. This could fit a 2k movie. However, 4k is 3840 x 2160 (this really is a full HD @2x), and again it would not fit comfortably in a @3x iPad although the pixel count is somewhat similar.