I've tried BetterTouchTool but found it a bit buggy. However, I have found another way to do App Switching that makes use of the otherwise useless Mission Control 😉 and is nearly as good as the original.
I've been playing with this method for a couple of hours now and it seems like the fastest/smoothest solution so far. Try it out and see what you think.
Basically, because Mac OS doesn't allow you to run more than one instantiation of an Apple programme, you simply put all your most used Apps on the first page of Mission control. When you want to app switch, you use the four finger pinch and tap the app you want to switch to. If the app is already running it'll take you back to the screen and windows you were last working on. If its not, it'll launch the app.
Of course, you'll have to spend some time laboriously moving all the apps around in Mission Control first so that all your most used apps are on the first page. My day to day work is usually switching between the same half a dozen apps, so with this line up on the top row of page 1 in MC, it seems to work pretty well (obviously, there's nothing to stop you swiping to other pages, too, i it just slows you down a bit).
There is one fatal flaw, however: Finder does not appear in MC. Any way to get it to appear there? (At the moment I'm trial running a third party extension to Finder called TotalFinder, and using the 'Visor' option as a work around, biut its not very satisfactory).
Changing the topic somewhat, this is the first time I've ever needed to use so much 3rd party stuff on Mac OS. Time was when "it just worked"). 😟