I've got a 5D3 and a 7D and was thinking about swapping both for a 1D4 with a guy who has one nearby who wants a 5D3, but after much consideration I've decided to stick with what I've got. The AF system on the 5D3 is magnificent and the IQ is just fantastic. I'd maybe give in to an offer of a 1Dx but that's about it.
If I could afford to add a 1D4 in place of just my 7D and still keep my 5D3 then I'd have the 1D4 like a shot and willingly give up the extra reach of the 1.6x crop for a 1.3x.