STS-9, also called STS-41A,
was the first flight to use the three-digit flight number system.
The first digit refers to the fiscal year - 4 = fiscal year 1984.
The second digit is for the launch site: Kennedy Space Center is "1", while
Vandenberg AFB is "2". The third digit is to indicate the order
in which the mission was originally scheduled during that fiscal year.
STS-9 was the first mission scheduled during the 1984 fiscal year.
NASA kept this naming convention even when the actual flight order of the
missions differed from the original schedule. In 1988, NASA returned to
the original naming convention with STS-26.