Oops. Didn't look deeply enough.
"Compatibility with Current FireWire Products
FireWire 800 provides two modes of transmission:
1. The pure beta mode (a FireWire 800 device connected to a FireWire 800 port with a FireWire 800 beta cable)
2. The backward-compatible legacy mode (a FireWire 400 device connected to a FireWire 800 port or vice versa with a FireWire 800/FireWire 400 bilingual cable)
FireWire 400 devices use a 6-pin or 4-pin connector while FireWire 800 devices use a 9-pin connector. Existing FireWire 400 devices can be plugged into either type of port with the appropriate cable.
FireWire 800 devices can achieve FireWire 800 speeds only when connected to a FireWire 800 port."
So 400 is electrically compatible with 800 but not physically compatible.