Great, thanks for the clarification. The manual says (for multi-track recording)
Data rate = 144 KB/sec per track (max 4GB total)
I had assumed that “total” 4Gb meant for all tracks, not just per track, because the 4 hour maximum given for stereo (2 track) recording using the same data rate implies a reduction to 28 minutes if recording 8 times as many tracks, which is in line with the calculation using the formula given above for multi-track. Ambiguous I think.