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I have the Canon 300 F2.8 L IS Mk1 but I do not have a 5D4!
I do have a 1DX, 7D2, EOS 3, EOS 33V etc - they all work superbly with my Canon 300 F2.8. I would have no concerns about using one with a 5D4. I notice you have the Mk2 so the excellent performance of the Mk1 with Mk3 extenders will be even better with your lens and a 5D4.
I have used a 5D4 on a EF300mm mk2 (and indeed EF500mm), usually with a 1.4x or 2x and have found it to perform perfectly okay, but personally having a 1DX2 as well I much preferred the photos from that. Focusing is also somewhat better on that as well in my opinion - logical given the battery size, hence why I ended up selling the 5D4. Given the higher density of pixels in the 5D4, your holding technique needs to be spot on as well as any slight movement is amplified more I feel.
If you look in my Flickr photo-stream and find the Nuthatch, Bullfinch and Blue Tit photos (they are all together) they are taken with the 5D4 and EF300mm and 1.4x in a dark woodland setting, so will give you an idea of how the combination performs - they were all hand held with me resting on the hide window ledge.