Heya,
I think you're splitting hairs at this point. The 1D4, 7D2, 1DX, etc, are all top tier cameras for aggressive AF and fast FPS. That's what make them great for birding. Trying to figure out which is top is a personal thing for you, there's more to it than just those two properties, like weight, feel, ergonomics, etc. Some will favor the 1D4 over the 7D2. Some will favor the 7D2. Some will favor the 1DX. They all shoot differently, different glass, and produce different images.
Personally if I were shooting a 1D4 I would only consider moving to the 7D2 if I felt I needed a little more pixels on my subjects due to distance to subject limitations based on the lens being used. Moving to the 1DX for example, would only be an option in my mind, if distance to subject was not a limitation and/or the length of the lens was also not the limitation (influencing distance to subject).
So unless you're already shooting a 400 F2.8L or 500 F4L, I would not stress about which camera so much at this point between those three, and instead, look at what glass you're shooting with and focus there more (again, unless you already have top tier birding glass). The lens will make a lot more impact if you're shooting something less than that, compared to the differences you'll see with those three bodies. Example, coming from a 400 F5.6L prime or 100-400 zoom to a 400 F2.8L is a massive tier jump.
Very best,