Testing telephoto lens EF400mm f/5.6 L USM + Tamron SP 1.4x white teleconverter at my backyard.
I removed the tape that cover the 2 pins (that I used in 50D camera, so that fool the camera for center auto-focus-non reporting aperture data).
Canon EOS 5D Mark IV have no problem with Tamron 1.4x TC, excellent AF at f8, not only in center but all the 21 cross AF points.
Bulbul Single, ISO 6500, 1/640sec, f8, focal Length: 560mm
Murai Kampung, ISO 400, 1/400sec, f5.6, 400mm
Bamboo (Buluh), ISO 1600
BulBul Pair, ISO 1600, 1/320sec, f8, focal Length: 560mm
Bunga Raya, ISO 3200, 1/60sec, f8 focal Length: 560mm
Waterhen (WakWak), ISO 1600, f8, 1/125sec , focal Length: 560mm
Juvenile Tree Sparrow, ISO 1600, 400mm, f8, 1/640 sec