I believe the reason for the limited number of onboard MFD selections was first and foremost predicated upon previous copyright issues, though I could be dead wrong about this. If you recall, the old MFD titles were all cryptic, thus skirting copyright laws.
Not many people actually Yamaha's MFD, many of which never take the time to explore its possibilities. Those of us that do use it consider the MFD a valuable tool, especially for live performances. The T4 MFD can access user files from the USB or HD, which adds to the versatility of the feature. Records are very easy to edit, and essentially, the MFD is nothing more than a miniature version of a registration. In my case, I created my own, custom MFDs, some of which have more than 600 songs. I use it in conjunction with registrations, thereby cutting dead time between songs down to a second or two at most.
Have fun with that T4--it's an incredible machine,