The R4 would have to support SDHC in hardware to be able to read anything from an SDHC card. Even if it was possible (which it isn't) to put some driver data on an SDHC card to make the R4 read SDHC cards, it still wouldn't work because the R4 can't see any of the data on the SDHC card. SDHC cards use a different method of arranging data on the card and it REQUIRES that the hardware knows how to access it.
If the R4DS was hardware flashable, then it would be no problem to add SDHC compatibility, but it's not hardware flashable. The R4 uses "software" firmware loaders, not a hardware flash to update the firmware. When you update the firmware on an R4DS, you just changed some files on the microSD and the "firmware" is just the first data it reads from the microSD when you boot the R4. It does not actually make any changes in the R4DS itself when you change firmware loaders. The R4's hardware is not flashable....it never changes, so there is no way to add SDHC compatibility to the current version of the R4DS.
Possibly, in the future, the R4 team will release a revised R4 that does support SDHC, or is hardware flashable, but the current design just cannot read SDHC cards, no matter how much we would like it to. I believe the fact that it can't support SDHC will be the R4's eventual downfall.
I'd love to have more than 2gigs available instantly on my R4 too, but it just ain't gonna happen in the current revision.