Upload & Sell: Off
I am with the others. If money is not a big issue, goes for the Mark IV.
I own both Mark 3 and Mark 4. In term of Image Quality, I like them all. But for small birds or bird with distance, Mark IV give you more pixel to crop. With the AF, I like the Mark IV. Also, the Mark III has only 19 selectable AF point where as Mark IV has 39. I find that for some bird composition, 39 points give me more choice to lock it fast when birds are perching.
AI Servo of Mark IV is pretty fast even with the busy background. Mark III is no doubt a darn good Pro body but I sold mine and just keep Mark IV for now.