I just bought 2 Sony SDXC UHS-1 70 MB/s, Class 10, 128BG SD cards. They were initially formatted in ex-FAT and weren't recognized by the Model RA4 radio.
I reformatted one of them in NTFS, added 202 albums/folders (over 2000 tracks @ 26GB). Everything seems to be working fine. Album art is showing up for the albums I've played so far.
It does not seem to be rebuilding the albums list after turning the vehicle off and opening the door, so that's a plus. I know I've seen some complaints that the list repopulates each time. I guess I'll see if this behaves differently over time or after I change to Sirius and back.
I tag/name all of my "Jeep Music" albums as "band name-album name" since I searched by album on my last 2 radios (RER & RHR I think), so I kept them the same. Plus at 200+ albums, I can't always remember the band just by album name.
The only slight difference is that the older radios allowed an ABC search by the first 2 (or more) letters. The current Model RA4 jumps right in with the first letter. Though it looks like I can use the regular "search" function for more than one letter.
In short, so far so good.