it is possible to get up to 4 inputs via firewire on any "desktop" computer, but be aware, any firewire video device will take up most of the available bandwith on your computer, you can plug in 4 devices, and all will be recognized, but only one will display video if all 4 devices are 1) daisy chained or 2) plugged into the same firewire hub
by defult a macpro only has one hub for its 3 firewire 400 ports, thus you can only plug 1 camera on there, (beware of other devices such as a m-audio firewire card which will also dominate firewire 400 bandwith)
the trick is to get 3 more firewire cards and install them in the macpro, then you can get your 4 cameras, or as mentionned earlier, you can always use usb video ports for other cams (beware of video latency through usb though)
maybe via a firewire 800 to fw400 hub you could do something, but I believe the firewire bus only allows one video device to be patched through
ahhh, the joys of video constraints.... if ever anyone finds a workaround, please advise, I've been dying for a multicamera package for onset work... it is truly bothersome to have to plug in 4 decks when you could have just one...
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Costa