The established standard maximum for non-SDHC SD cards is 2GB. There IS a such thing as a non-SDHC microSD that is 4GB, but it is very hard to find and is very slow....not to mention that it doesn't conform to established SD standards and may not work properly in many card readers or devices. While the R4 may technically support such a card, its speed would likely result in many games crashing or hesitating on you.
The R4's with the new shell style are essentially the same exact card as the old-style R4, but the spring was removed from the microSD slot because of issues with the spring, and the casing style was altered. Neither the old nor the new style R4 supports SDHC cards.
There is a rumor that an SDHC version of the R4 is coming, but nothing concrete has really been announced yet.
At one point it was thought that a firmware update could possibly add SDHC support to older R4's, but that has proved impossible because the R4's hardware isn't flashable and the latest update (1.12) didn't add the ability like we hoped.
In short, 2GB is the MAX size you should look for in a microSD for the R4. Kingston (Japan) is the recommended card of choice for a 2GB.