Once again check the write speeds between the 4gb SD and SDHC and the classes of speed. If you're only going to use CHDK for taking jpeg pictures than you shouldn't have a problem with either card. But if you're going to be taking RAW or DNG with the camera then I suggest a fast card as this takes more time for the camera to write the info (as it records both jpeg+raw/dng). Having a faster card would help speed this process up.
If you go over 4gb for example 8gb or 16gb card then you will need to format it and make two partitions with the card in the camera, install CHDK on one partition, then having to swap the partitions in the CHDK menu so that CHDK can autoboot.
If you're using Windows and want to import your pictures from your camera by inserting the SD card into your computer, you first have to "swap partitions" under "Miscellaneous stuff" in chdk or windows won't find any of your pictures even though your camera can review the pictures, its like they are hidden.
Once you have imported your pictures and you want to get CHDK to autoboot again (it requires another swap partitions) you have to select the firmware update in "review mode" in Canon's menu system. Run the firmware update to get CHDK started then hit the "print" button on the camera until you see "ALT" at bottom of screen, then menu button to finally get the CHDK menu system. Go to "Miscellaneous stuff" and choose the "swap partitions" options to get CHDK working in auto mode again. When you do this process once its a piece of cake after to remember.
I haven't tried hooking up the camera direct with a usb cable as yet to see if the camera reads the pictures without the swap function you can give that a try.
Cheers.
