I don't know if a 4GB card will work, but it's interesting that Roland now officially lists three 2GB cards in their compatibility chart
(Google-translated from a japanese page
). Of course, as cuthbert pointed out, the Micro BR can only address 1GB of memory, regardless of how big the card is.
Be aware that many 4GB cards are actually SDHC rather than SD, and I doubt that SDHC will work in a Micro BR. So, if you're going to try a 4GB card, make sure it's the older SD type rather than SDHC.