(Adds details, quotes) By Martyn Herman RIO DE JANEIRO, Aug 16 (Reuters) - Britain's Laura Trott won her fourth Olympic gold medal with a masterful ride in the track cycling omnium on Tuesday, leaving her rivals scrapping for podium places. Trott, the first British woman to claim three Olympic golds when leading the team pursuit home last week, held a commanding lead going into the points race, the sixth discipline of the event, and turned it into a procession. As the 24-year-old ticked off the 100 laps without incident, picking up early sprint points for insurance, the real battle was for silver between American veteran Sarah Hammer and Belgium's Jolien D'Hoore. More...