Now that the price of memory card have plummeted it is tempting to get the bigggest possible.
8 GB is now almost the rule. I want to say that there are problems with "my card is bigger than yours" like boys in the locker room.
For a start not all devices will take these super large cards and can produce unnerving error messages.
Just imagine that you have a 4 GB which if you have a 10 Mega-Pixel camera it will store over 500 pictures and you lose it; for me that will be about a fortnight's "snapping". Also technology is not always perfect and they can fail.
I recommend 2x2GB rather than 1x4Gb is a much safer route. It also encourages good house keeping of your card as your are more likely to download them on time and not leave them suppurating in the camera!