I do think that Canon have been too conservative with the memory card slots (the jibe of "welcome to 2012" is not unwarranted). Sure, as long as it'll use the fastest UHS-I speeds (of around 100MB/s, which is 800Mb/s) then either/both cards will be fine for even 30fps 4k MJPEG (60fps would be a stretch on the SD card, but the 5D4 doesn't support it anyway).
However, it is an issue for stills. The sub-$2000 Nikon D500 claims up to 200 raw images, and has a UHS-II SD slot (as well as XQD), so the 5D4 looks pretty poor in comparison.
The 7D got a firmware update that increased the buffer size, and the cynic in me suspects it was less about better optmised memory handling, and more about increasing a constant in the code that would allow it to use a bit more of the internal memory. Memory's dirt cheap these days, and even with 40MB raws, a paltry 1GB of RAM is enough for 25 shots (before you even touch the card). The fact that a number of Canon's bodies now have apparent buffer sizes that are good for exactly 3 seconds of shooting makes me suspect there's probably more RAM available - it's just limited by a constant in the firmware. I mean - it's not like we're griping about 5 vs 6 shots; it's 21 vs 200 (even allowing for the 50% greater resolution of the 5D4, the D500 buffer is way higher, and would be able to empty that buffer to the cards way faster).
I am actually surprised that more company's don't use the XQD cards. With more file size and video all the time it makes sense.