Between 2 & 3 will be the normal operating levels, 1 will be overfilled, 4 will be too low. 1ltr every 1000 miles is the maximum permitable useage of oil, actual oil use is likely to be way less than that, more than that and you have engine issues.
Further to this....where will the oil be when the warning light comes on (not had chance to check, now it's dark)? Mine came on today and I put a litre in less than 1000 miles ago, im pretty sure. It's 1000 at the max. I'm gunna ring BMW tomorrow, its under warranty with them atm.
I believe the book says to check within 5 minutes after shutdown once the oil was at operating temperature. I think once the engine is properly warmed up.
As long as there is some oil on that middle bit a short while after switch off and when warm, then you are ok.
If it's looking in that very bottom section, best to warm it up, park it dead flat, wait 5 mins, then dip a clean stick and get a more accurate measure to be fully sure where it is, then top-up from that if necessary!
My car used about 1l of oil in 6,000 miles, so not bad at all really. Every 30,000 miles you have effectively had an oil change in top-ups alone I'd say