Are you running Jellybean or ICS? When the droid 4 came out they claimed it was only good for SDHC. I was wondering if the jelly bean update fixed that.
Running Jellybean now, but bought it right after I got ICS. I don't think SDXC was really out when the official spec sheet for the Droid 4 came out.
My understanding is that SDXC uses the same hardware as SDHC, so most SDHC devices support SDXC cards even if they aren't spec'd for it.
I have a 3 year old MicroSDHC to USB adapter, and it works fine with SDXC cards.