I just added a 5D IV to my 7D II to shoot birds with my 500 F4 II. When I did some controlled "croppability" tests, I found that the 7D II still out-resolved my 5D IV, but that with a 1.4x TC, the 5D IV beat the 7D II. The order was...
- best was 7D II with 1.4x
- second was 5D IV with 1.4x
- third was 7D II bare
- fourth was 5D IV bare
The 5D IV is still very croppable though.
That's pretty much what I have found so far, although I haven't had much of an opportunity yet to really try them both out on the same exact type of target. I think I will keep my 7D2 as the primary birder with the 500 f4 and use the 5DIV with the 100-400II with or without the 1.4 on it. I do sometimes get too close for the 500 and its nice to have the other camera ready to go.