4K or UHD (not quite the same as 4K) resolution, even if 'trimmed' to the usual formats needs a horizontal pixel count of 3840, 3996, or 4096 depending on format (roughly 7-9MP depending on the matching vertical dimension) which the iPad Pro 12.9" is a long ways away from (about 5.6MP). If you start trimming it from both directions it's not really 4K anymore
4K on a 12.9" 4:3 screen would deliver a ppi of 397 and the iPad Pro 12.9 is only 265ppi. 1440P (2.7K) is usually 2560X1440 (technically it is 2704X1520) which the iPad Pro matches almost exactly in the horizontal dimension. The screen itself is quite a ways away from 4K resolution even considering it's aspect ratio. If the iPad Pro 12.9" had a 4K screen, the resolution with it's aspect ratio would be 4096 X 3072.
If the iPad Pros were doing anywhere near 4K resolution, Apple's marketing folks would be all over it