Actually, the Fox RP23 sits between the Monarch 4.2 and the Monarch 3.3 in terms of features. Both the 4.2 and RP23 have a dial adjustable platform, with a switch to turn the platform on and off. Fox call the platform Pro-Pedal, Rockshox call it Motion-Control.
The Monarch 4.2 has about 12 clicks of adjustment, where RP23 has only 3 clicks of adjustment (light, medium and solid platform)
The Monarch 3.3 has slightly less adjustment than the RP23, having a single lever to switch from No-platform, to medium platform, to solid platform.
The monarch 3.1 has just got a dial to vary the amount of platform, and no lever to turn the platform off altogether.
Personally, I'm not a big fan of dials with hundreds of clicks to choose from. I bought a Monarch 4.2, and kinda wish I'd saved a couple of quid and bought the 3.3 instead.
Just realised I've answered a question from June last year :-s that was worth the effort. :-(