Arlo,
your ScanDisc Extreme III 4 GB card is a High Capacity card everything bigger than 2 GB is High Capacity (HC)).
So the Humminbird Card reader you're referring to will NOT work with it. Go the next electronics or PC shop and get a SD card reader that is capable to handle your card. They aren't a lot more expensive than the regular (old) SD card readers.
As for the AS-PS connection kit:
You will need the AS-PC cable from Humminbird to connect the unit to the computer, but the power source wiring that comes with the AS-PC connection kit might not be sufficient for powering up your unit for an extended time.
You better cut off the battery holder and connect the wires to a decently loaded and working 12V car or motorcycle battery. Don't forget to put an 3 Amp in line fuse into the plus (red) wire. You'll be in a world of trouble if you're updating your unit and the update is interrupted due to a power loss during the update.
If your PC doesn't have an RS 232 (9 pin) / serial port but USB ports only you'll need a RS 232/USB adapter.
You can get them at PC shops as well or you can get the Humminbird one:
http://store.humminbird.com/products/322963/AS_USBHope this helps and good luck
Regards / Harry