I'd be happy to help in the electronics department if it's needed... I do lots of custom circuits in my spare time (Meche by trade EE by hobie)
as for the splits...
if you're going to split it PERFECTLY 4 ways you're looking at
5% - 20% - 35% - 50% that a clean 15% change with every "click"
however I get the impression most people are really only looking for this:
5% - AUTO - 50% where "auto" is simply letting the TOD do its thing, 5% gives you the best fuel eco and 50% gives you the best traction.
if you wanted it to be really snazzy you could go with "wheel" type adjuster (think like the scroll wheel on your mouse but tighter to hold itself in place) that "clicks" on both ends... meaning wheel it all the way down and click for 5%, wheel it all the way up and click for 50% and then infinite adjustment in the middle... you could have a 2nd switch that would allow you to switch between TOD control and whatever you have the wheel set to.