So I guess someone read my statement, LOL
Wonder if it would also work for the 5DIII.
If you read Art Morris' article, you'll find the answer. Hint - if you're a 5DIII owner, you won't like the answer. But it's there.
I tried it with the follow;
1DX with EF400L 5.6 plus Kenko 1.4x Pro DGX - f/8 - OK - all AF points works - AF is somewhat front focused.
1DX with EF400L 5.6 plus Kenko 2.0x Pro DGX - f/11- NO GO - Hang-Up - Had to Remove Battery.
1DX with EF400L 5.6 plus EF 1.4X II - f/8 - NO AF but Camera WORKS
1DX with EF400L 5.6 plus EF 2.0X II - f/11 - NO AF but Camera WORKS
5.6 plus 2x = f/11 - there's no camera that AF at f/11....
you can manual focus though on bright daylight
My 1D Mark IIN and your 1D Mark IV will work on f/8 but not f/11 - none does
hope this helps your inquiry.