I have all my class 4 SD cards (including one micro SD) work perfectly for me.
Should I encounter problem on reading the card (TP's blue light illuminates continously), I've those folders re-rip by Max (for Mac) and it will do the trick and since then TP can play all songs.
Here's the Max software that I use for ripping CD & conversion from Flac to WAV.
Here are my class 4 SDs all working perfectly. I've found there is no need for high speed card even with 192 high res files and that saves you a lot of money!
Are your class 4 SD cards from Sandisk? My guess is they are not and I think that may be why they work fine - because if they are Sandisk class 4 then I'm completely confused since I have multiple other cards (all class 10) from Sandisk and Transcend and the exact same folders/files (all ripped by XLD on Mac) that won't work on my class 4 Sandisk (continuous blue light) work perfectly fine on all those other cards. By the way, all my cards are formatted the same way using Mac disk utility (for FAT32).