Jeez. If the engine cannot run with 4.5l of oil in it there must be something wrong somewhere!! In theory if there is oil in the tank, i.e the scavenge has done its job and pumped it back, then there should be oil for the main pump, to pump the oil around. The only problem I can see with running too low an oil level with a dry sump is, providing the scavange is pumping faster than the main pump, the oil will get too hot. But hell, my Vx runns about 3.5-4l with no problems thus far.
I hope that makes sense.
It wil be interesting to hear what cosworth say. If the ECU has logged periods of low pressure, at somepoint the main pump has been starved. 1l differnce in the tank should not be that much of a problem. So it would suggest that the scavenge is not doing its job, or the main pump pick up is not effective enough to keep up with the G loads in corners. IMHO of course.
RED 2.0 HPC 230BHP here