In your boots, I'd only look at the 7D Mark II. So, if you buy a 1D Mark IV, you'll see about the same camera as the 7D mark II. Many of the same bells and whistles. But, you'll have bought a used camera, with all the problems it may come with. I looked up what a used 1D Mark IV would cost on B&H, the average was about $3000.00. You'll get no history of the camera, except it is "used", how well used is anyones guess. It is a professional camera, and probably used by a pro, in a studio or in the field. Again no record of its use, or abuse. How many times has the 1D Mark IV been sent into Canon, or anywhere for repair? Totally unknown. As I said, "In your boots" the 7D Mark II give you totally modern Canon technology, new focus system, new A1 Servo, and much better focus system. Plus, everything is new. The body comes with the paltry Canon 1 year warranty, the shortest in the industry. But, you can be sure Canon will repair anything on that new 7D Mark II, for several years to come. The 1D IV is completely out of production, how long Canon will repair the camera is anybody's guess. The next shot of the 1D IV may be its last. I'm not a gambler, so I'd go for the 7D Mark II, and walk away a winner.
B