I think you mean GB not MB ?
The price difference could be that the 2GB is a "Fast" read/write card.
And the 4GB is a slower read/write card.
So depending on what it's to be used for, there could be a longer delay period until the hardware is ready for use.
IE:
If it's for a SatNav, then you could possibly miss the next turn as the directions for the next turn are not ready to be announced.
If in a camera, again the time before being able to take a sequence of pictures would be hindered by the time lapse the camera needs to write the last shot to the card before being ready to take the next shot.
Unless it's only the card you need, then personally I don't see the point in having to mess around with the adaptor/holder, which is then inserted into the equipment any way.
Too small and fiddly.
