Hi,
I agree with Todd, we have a couple of Analog way smartfade units which have a pc based interface for configuring inputs and for switching, we tryed in vain to get the switcher to talk to a mac, we used a variety of usb / serial adaptors which work for other serial devices with macs without issue, we ended up having to use a pc to get the software config tool to work, never worked for us on a mac. Another way to go might be to get hold of one or more active silicon 4 input cards, i recently bought one and am really happy with the results, there is very minimal frame delay on input and you can preview each input by opening the inputs window in catalyst so you can see live what cameras have a decant picture.