What is the capacity of a 2g SD card at 6.1 megapixals? Can I use a 4g card in a Dig. Picture frame? Thanks?

Even though the frame says the max card it can read is a 2g, I was wondering if I could use the 4g. The cards are the same footprints so if it fits would it work and which type of memory card holds the most pictures, the frame is able to read several different cards, so is one better then the other. Thanks!!!