I'm a little new myself - I have a M3 Mini SD. From what I can tell the 1x, 4x, normal all work the same for me. I use a RiData 150x speed mini SD.
Everything loads fast and plays without a hitch. I'm actually using FAT16 at the moment because I heard it might help something which I forgot atm.
You'll most likely be perfectly fine with FAT32 and if it causes a problem later just back up your card - reformat and dump all the data back on.
BTW - I've seen 4x work for every game so far except Megaman ZX - which for some crazy reason glitches when you pick up and use the 2nd (green) suit. 4x works perfectly every other time though.
In megaman I used "safe mode" or something.