It is my understanding that if it is 1/4 qt low, 3 bars would be left showing instead of all 4 when it says ok.
I had an '07 X3 that the dealer couldn't seem to get filled right. I do know that if you put too much, it will show "O".
2008 Space Gray Z4 3.0 si, Saddle Leather interior
2011 Nissan Frontier King Cab
2010 Lexus RX350