The topic is mixing 10 speed FDs with 11 speed drivetrains. There are a couple different versions of 10 speed FDs, since some are QS compatible and others are pre-QS. The QS models will definitely require 4 clicks if used with the narrower 11 speed chain and I would expect that a 4-click setup may even be required with a 2009 or newer 10 speed FDs, due to both a wider cage and narrower chain.
All 10S FD share the same cage width and that's all that matters here. The only possible problem you could run into is when using a pre-QS era standard double only 10S FD on a 11S CT system.
Setting up a 11S or 10S FD for 4 clicks, or 3 for that matter, without having the stops adjusted correctly is just asking for trouble and will sooner or later lead to chain drops.
Allowing for a fourth click only makes the derrailleur move in smaller steps. The total amount of sweep will always be limited by, what else, the limit screws.
A lot of users setup their 11 speed FDs with 4 clicks, just because it takes extra effort to dial in the limit screws and get just the right cable tension to use only 3 clicks. Using 4 clicks, just takes up cable tension that could be added with the barrel adjuster.
Well once you start playing outside of the ballpark anything's possible....
Point is however that when using a 10S FD within a 11S system you'd better make sure everything is set up as it should unless you don't mind the occasional chain drop..
For that, and that only, is the reason why the width of the FD cage was reduced.
Some frames don't even have barrel adjusters on the frame. Without adding an inline adjuster, that makes getting the cable tension correct, particularly tricky. What I do is deliberately turn the low limit screw CW by 1/2 to 1 turn, pull the cable tight and then tighten the cable clamp. Return the limit screw it's proper position and with luck, you'll have the needed cable tension.
Sure, but that works for all derailleurs regardless of them being 10 or 11S. The higher the cable's pretension the more it's pulled away from the frame but the stop will still need to be set correctly with the cable in a relaxed state.
@thisisatest: They could be set up for 3 clicks but there was little point in doing so as the LH shifter lever wasn't moving 1:1 on the first step so it didn't make it much sense.