Yes, they are compatible, but you can't always get the most out of them.
Any Synergy module that is Fender biased will work perfectly well e.g. TDLX.
Any one that is Marshall biased will work less well - anything Friedman, Plexi etc.
Any one that is 'boutique' biased, such as SLO, won't sound very good.
The fix is to get a bias switch on your RM4. Jaded Faith can send you a kit. It's not expensive. If you want to play your Plexi module, flick the switch on the RM4 to the Marshall setting.
However using the footswitch you won't be able to automatically adjust the bias between say a Fender module and a boutique module.