4 Gig cards
The only 4 gig cards that work with the M8 are the cards recommended by Leica. I have the Hama cards and they really work well. The reason why the others dont work is simply that the other cards are SDHC which are physically different and have a different structure. In theory you should not be able to get 4 gigs on a standard SD card (2gigs was the limit) but somehow Hama found a way and it works.
I now use larger cards because I hate opening the baseplate in the open - yes, I hate changing lenses too! I was out in the desert in the Gulf recently, the wind was blowing and I changed lenses. I have spent the last few weeks taking dust spots off images and plucking up courage to clean the sensor.