Do I understand correctly that you gave it back? I would have been interested to figure out what's up with the card and maybe even offered you to exchange with mine... but to answer your question:
The 4GB-MMC-plus I use is from "Pretec" - it works fine in my OndioFM but it's slower in write speed than my Transcend 1GB-MMC-plus. This is most noticable when you copy many small
files to it (like .txt). If you only copy average mp3s there is not a big difference, especially when using the Ondio as card reader. And I could try another 4GB card from "A-Data" which is even a bit faster, if I remember correctly.
P.S.: If no-one answered you, please use the "modify" button and edit your own last post instead of writing a new one. The forum guidelines ask you to and you agreed on following them during the register process.
* December, 5th 2007 *
Followup on this old thread in case someone stumbles upon the same problem:
There really was a problem in the MMC driver which could not handle these specific Transcend cards (the 4GB one and the 2GB as well, I think). The problem was solved in revision 18515 and some small following optimisations to the MMC driver were made until r18552. - This fix should also be in the 3.0 release build.
One hiccup with the Transcend cards, which violate the standards, is left though and might prove impossible to fix: Rockbox will not find these cards when they are already plugged in at boot ot after disconnecting from USB. But hotswap is working so if you pull them out a bit and push them back while Rockbox is still running, it will work and is far better than not reading the card at all.