I can understand a delay of an hour, maybe even two, being due to delays at airports, but four? That's a bit much. Many airlines build the expected flight times and delays into the schedule so that, for example, on a BA flight from Jersey, that highly congested international airport (!) to Heathrow, the flight time would be 35 minutes, but BA builds in an extra 25-30 minutes, to allow for holding. Similarly, airlines flying from Dublin to Heathrow usually allow a "block time" of 1h20 for the 55 minutes flight.
My worst ever delay was in 1981, when I was returning with my family from Kuwait. The flight was delayed by at least two hours. No explanation was ever given, but in retrospect, I suspect it was due to the temperature (about 52 degrees! - it was late June) being above the maximum for the aircraft's weight on the day.
Did the good people of CO explain why a 777 flight could be delayed over 4 hours?