As Rob above wrote, you just buffer the data for a while and make sure no measure is lost. If I understood it correctly, at worst you have 4 simultaneous lap times and then at least one second before the first intermediate time measure can occur. That's long enough to send out all data over the serial port at 9600 bps in your proposed format.
The Arduino Uno is good enough for this, there no real need to a Mega.
Do you also have a display board and starter lamps installed and how are those driven?
Korman