For the meaning of BDF click on:http://www.sailing.org/tools/documents/RRS2009-2012-
Rule 30.2 of the RRS
30.2 Z Flag Rule
If flag Z has been displayed, no part of a boat’s hull, crew or equipment shall be in the triangle formed by the ends of the starting line and the first mark during the last minute before her starting signal. If a boat breaks this rule and is identified, she shall receive, without a hearing, a 20% Scoring Penalty calculated as stated in rule 44.3(c).
She shall be penalized even if the race is restarted or resailed, but not if it is postponed or abandoned before the starting signal. If she is similarly identified during a subsequent attempt to start the same race, she shall receive an additional 20% Scoring Penalty
Since the 4th race was completed after 9 p.m. I can understand the reason why the Racing Committee hoisted the Z flag