Don't quote me but I think North Weald in June 1982 was to have been the first opportunity to see all four prototype Routemasters together. Unfortunately RM3 was unable to attend owing to a mechanical defect so the chance was missed. Here are RM1, RM2 and RMC4.

The four prototypes were subsequently gathered together at the Chiswick & Acton Gala in July 1983, where they were joined by a certain "RM5" and RMs 6, 7, 8, 9 & 10. (NF)