Does the 4200X recognize SDXC micro cards or just SDHC?
Does it work with MicroSD cards formatted as NTFS, exFAT, FAT32?
What file system is native? (What it uses when you format the MicroSD with the Roku.)
I can't answer re SDXC.
I do know the Roku will only use 2 G of the card, no matter how big the card is. It only uses the card for expanding channel app storage, not as system memory. The Roku should recognize a FAT32 card (don't know about other formats) and offer to reformat it to whatever format it actually uses.
It formats it with some kind of encrypted file system. I don't know the answer about SDXC either, but it would be a waste. An old microSD card you might have lying around from years ago is just fine. Don't expect any kind of performance improvement. The card is only used to store apps so they don't have to be redownloaded. It's often more trouble than it's worth since a corrupted card will cause all kinds of strange unexplainable problems.
I have a Class 4 4GB Lexar MicroSD from back in the dark ages so I'll use that.