Hi Kelly,
If the mulch is truly from a recycled source, then yes it can count towards MRc4. Any mulch from a virgin tree cut for the purpose of being mulched would not count, but if the manufacturer can display that they are re-processing a material in compliance with ISO 140221, then it should count.
I'm assuming that you have trees on your site that are being cut down to make way for a building? They would be included in your MRc2 calculations, and chipping them to reuse on site certainly seems to me like a reasonable manufacturing process to qualify as recycled content.
That said- it's possible that the GBCI reviewers could see this practice as akin to the use of crushed rock that has been excavated on-site. The crushed rock can be applied to MRc5, but not MRc4. So- include the recycled wood mulch in MRc4, but be prepared in case it is turned down.
Best- anne