The R5 will have to use a different codec than the 5D4 for video. My guess with HEIF image suppport, H.265 for UHD (which is now a standard HEVC video format).
I highly doubt Canon will give H.265 (or H.264) codec for 8K, it will be the crippled mjpeg like the 1Dxii and 5Div for first roll-out of highest video resolution. Canon did say that it'll give me an introduction to 8K.

