If you use pedals and such I would recommend a GCX unit to control effects and keep them at the right levels....A GCP is a nice addition but that's just a controller pedal and obviously doesn't go in the rack...But then, you'll be ready to control your next couple RM4s as you add them 8)
I would recommend looking into the rack version of the ISP Decimator GString to keep your rig quiet...Although the Gstring pedal is cheaper and does the same thing (only one channel instead of two)...I know everyone raves about the rack unit...
I'd also consider a good EQ...Don't have any suggestions on what kind...
If you get a power conditioner (and you should) make sure it has a jack for a rear BNC lamp so you can see what you're doing back there....Especially if you're gonna gig with this...
A rackmount tuner is great since you'll have the rack anyway....They are relatively inexpensive and have a much larger display than the pedals...I wouldn't trade my Peterson Stroborack for anything...It's always on and extremely accurate, fast and visible....
You may also think about a rackmount fan or fans for your gear...I don't use an intake one...But I do use a 3 fan setup blowing outward in the back of my rack...But this also means your rack will need rear rails...Most already have them...Rackmount fans are abundant and cheap on Ebay...
If nothing else, DON'T SKIMP ON BUYING A GOOD RACK...Especially if you gig...You're looking at a couple to few hundred bucks for a good one...Make sure you get wheels...Many don't come with them but rather sell them as add ons...Consider that in the price...
And +1 on modules...Seems I always need one last one...
Good luck!