I can't find any rules on Characters with Muscle Replacement and Cyberlimbs. I'd assume it would just be a 20% to 25% reduction in cost/essence for each cyberlimb character has when they get Muscle Replacement/Toner/Augmentation

Those are SR3 Rules.
Not in SR4 anymore.

Technically everything stacks in SR4 (skin mods, etc.). Feel free to house rule it logically.

Indeed, cyberlimbs are wacky with RAW. I tend to have them not stack at all and use the house rule of 10% essence discount on any and all full-body 'ware per full limb/torso/head the character has replaced. 5% for partial limbs, 2.5% for hands and feet.

This includes muscle/bone/skin enhancement.

Carefull . . you are veering of into hit locations there . .

Sounds reasonable for muscles, but I doubt I would add bone and skin. Bone replacements make sense, as they could be"updating" the structural capabilities of any cyberlimbs while they replace the skeleton. same goes for the outer layer of the limb when they add the skin. This of course is for cyberwear versions. Biowear it wouldn't make much sense for bone density augmentations to affect a full cyberarm.

Them's the breaks though . .
As i said, SR3 had rules for this kind of thing.

Just use cyber suite rules and you are set.

Nah, too easy, we need stuff to fight about here! ^^