I just noticed this thread. This is a response I posted to another thread. Hope it helps.
I have the 5D3 and 5D4 bodies both with the latest firmware. Here is data from a test I just completed:
Both cameras set to RAW, manual exposure, 1/5000, lens wide open, lens set to MF, lens cap on
5D3 w/Lexar Professional 133x card 15 shots until buffer fills, 26 seconds for buffer light to go out
5D3 w/Lexar Professional 633x UHS1 card 15 shots, 16 seconds
5D4 w/Lexar Professional 133x card 20 shots, 38 seconds
5D4 w/Lexar Professional 633x UHS1 card 24 shots, 13 seconds
I performed all the above tests twice and numbers were virtually identical. In actual use when shooting a couple series of RAW bursts my 5D3 can go into slow shooting mode from time to time when shooting RAW. I've never had this happen when using the 5D4. It just keeps on shooting. That's not to say if I (obviously) shot a long enough burst it wouldn't stop. That's just not real world shooting for my uses. Hope this helps.