I have both. The current EOS R does exactly that. In fact, it's better than the 5D4 in lots of ways for landscape shooting, which is what I do. Only potential concerns I have/had are:I'm a 5D IV shooter who wants a FF mirrorless with the same or better capabilities. I'd love to start using Rf glass, but I don't want to step down or buy a high-MP count sensor. Doubt I'm the only such customer. In the meantime, I can take great pics with the dSLR and legacy Ef lenses.
- weather sealing is good, but not as good as the 5D4
- second card slot for some users
So if it's really wet I use the 5D4, otherwise I'll take the R anytime. The files are no different than the 5D4 in terms of post processing latitude. The latest firmware solved the banding I read about but never noticed.
Despite what you read, I would have NO hesitation to getting an R, especially now that's it under 2K. (This is as a stills shooter; I shoot video on my phone so I cannot comment there)
The R/5D4 makes a great body combo right now, and I plan to slowly get a few RF lenses as they come out. No rush, as all EF glass works great with adapter. When the hi-res R is released, I'll switch over to RF totally and my R becomes second body.