Here's the story on 4Gb SD cards.
There are 2 types of 4Gb SD cards, SD and SDHC. SDHC will not work in a wizard at all. From what I've read, there's a possibility that sometime in the future a firmware update could come out that will allow it to read them, but unless I've missed something recently, that hasn't happened yet and may never happen.
That doesn't mean a 4Gb SD card will actually work in your wizard. So far, there seems to be 3 different groups, people that can't get a 4 Gb card to read in their wizard no matter what they try, people that get them to work but they constantly get corrupted and have to continually reformat the card about once a month (sometimes more often than that) and then restore everything on it (I actually belong to this group), and people that have no problems with them.
If you can afford to buy one and risk not having it work at all, then by all means buy one and try it out. There's a thread in the wizard accessories section on working 4Gb cards that will help you decide which one to buy. That's still no gaurantee it will work in your wizard, but it will give you a better chance that it will anyway.
My advice that I usually give though if you can't afford the risk is to buy 2 2Gb cards, copy the main files that you want to have always there to both cards and then split the rest of the stuff you want to put on there, such as MP3's on one card and movies on the other card or whatever it is you're wanting to put on them.
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