How can I gain the 'true' space on this micro sd? First off I know this post is about a micro sd, but I'm on a Mac..
Anyway, I got a micro sd card with my phone. It's a Cricket phone that features their Muve Music service. (Though I rooted and removed the app for it) Though the phone came with a 4GB Micro SD card, 3GB  of it was dedicated to Muve Music; and the other 1GB was for the user.. Is there a way to use my Mac to access the full 4GB?
I recommend you insert it directly into a sd card reader attached to your Mac and backup the part of the drive you can access to your Mac's drive. Then open Disk Utility.app select the SD card reader in the left bar on the top level of the hierarchy (not the volume itself on the second level!), switch to the partition tab on the right hand, choose "1 Partition" from the "Partition Layout:" menu and click on "Options" beneath it. Choose "Master Boot Record", confirm the dialog with "OK" and click "Apply". Wait until it's finished, then check if the size of the volume has increased and if it has you can copy your files back.