The 4X range were used by services originating from Chai Chee
Terminal in the 1970s. AFAIK, there were
numbers 40, 41, 44, 45 and 46. Eventually, all were withdrawn
except for 40. Later, 42 emerged when M2 was re-numbered. These 2
were the only services in the 4X range until the recent
introductions of 43 and 48.
