Maybe at 10 fps
I think the bottleneck is to read out and use the split pixels simultanously during the readout of ... 8MPix x 25 fps which results in reading 200 million pixels per second, measure their brightness value with 8...14 bit resolution, calculate the 8MPix images from the Bayer pattern, apply lens corrections etc. and encode the resulting pictures into a movie stream. Doing contrast detect AF is maybe done on one CPU core or special built in processing "circuitry" of the DIGIC 8.
The same with the 100 (120) fps mode in HD (1280 x 768) where exactly NO AF is available.
So I do not see any chance to have DPAF with 4k @ 25fps.
See also
https://cpn.canon-europe.com/content/education/infobank/capturing_the_image/digic_processing.do where they state that a lot of functionality inside Canons DIGIC processors is not software but hardware which makes them fast but energy saving at the same time. DIGIC 4 (5Dii) to DIGIC 5+ (5Diii) made the processor 17x faster which cannot be done by ramping up CPU clock or the number of cores ... enabling better on the fly - lens aberration correction and/or video quality