I have used this combination before, and it really depends. Most of the Kenko 1.4 TCs will allow you to AF without any kind of tape tricks, however only when there is enough light. If you have particularly bright lighting conditions, then AF will work...albeit very slowly. In any other situation, AF will be spotty at best, if it works at all. In lower light situations, AF will usually just hunt back and forth, quite slowly, without ever actually locking on to anything.

You need to make sure you attempt focus on high contrast areas, preferably obvious contrast (not just microcontrast). If you try to focus with the Kenko+100-400 on an area that is largely uniform or similar in tone, then AF will usually just hunt.

I would also like to point out that, even assuming you DO manage to get AF working well enough to lock consistently, the 100-400 just doesn't have the base IQ necessary. Slapping on a TC will greatly exacerbate corner and midframe IQ issues, and the smaller maximum aperture immediately limits your diffraction-limited center-frame resolution.

Overall, it is really not a worthwhile option to use the Kenko 1.4x with the 100-400. It is really not a worthwhile option to use any TC with the 100-400...it just doesn't really perform well or produce keeper IQ. You are much better off saving your money for a better lens (i.e. 300mm f/2.8 can handily take a 2x TC III and still produce better IQ than the 100-400 on its very best day!)

If you need a larger subject in the frame...the best bet with the 100-400 is to figure out how to get closer. When it comes to wildlife and birds, some camo pants and shirt will go MUCH farther than a TC towards getting you more frame-filling shots, without any IQ loss (rather, with an IQ improvement, assuming your subjects don't flee.)