Hi Molly,
It's important to be aware that there are two types of SD cards, SD & SDHC. SD is the standard card that's been out for quite awhile now and supports capacities up to 2GB. SDHC or SD High Capacity is a newer standard card that uses the same size card and connection but is compatible with a range from 4GB to 32GB.
Currently, the Leica M8 does not have the hardware necessary to interpret SDHC cards making the max capacity 2GB.
However, some card manufactures figured out how to fit 4GB of storage into standard SD cards before SDHC cards really caught on (so they weren't widely supported) and they were expensive to manufacture. This is why there are *some* 4GB SD cards that work fine on the M8. You can find a PDF from Leica (
link here) that lists officially supported cards and it includes the 4GB non-SDHC cards that work fine.
Now here's where things start to get really confusing... There is a user here, Gabriel, who has reported that he uses a 4GB card made by a manufacturer named "Patriot" that works fine. What's confusing about this is that the card is labeled "SDHC" and since the Leica M8 physically doesn't have the hardware to interpret SDHC cards it's really strange that it works. There are a couple possible explanations for this, including that Patriot might have mislabeled the cards, but no one knows for sure yet. Also, he has reported that he's only tried the 4Gb card, nothing in a larger capacity which would really prove whether or not the card is SDHC or not.
So, all that to say that right now using a 4GB card on the M8 is a bit of a crap shoot. There are some that work, and its best to just use the list that Leica provides to find an officially supported one if you are in need of 4GB. There's *possibly* some other ones that work, such as the Patriot, but all we have to go on is random user reports and without the official word from Leica just be aware that its not been officially tested or supported so something *could* happen like photos not recording properly, etc... There is also a slew (most) of 4GB cards that just don't work in the M8 at all. There also has been no single user report that I'm aware of of any 8GB or higher capacity card working in the M8.
Hopefully that's useful. Just be careful when buying a 4GB card and do your research. *Any* card 2Gb and under *should* be just fine.