Horrace is close, but not quite right. The first 100 numbers are reserved for Presidents of the AMA:
AMA 4 Irwin G. Ohlsson Sr
AMA 8 Frank B. Bushey
AMA 9 Keith H Storey
AMA 10 Claude McCullough (1957 – 1958)
AMA 11 Walt A Good (1959 – 1960)
AMA 12 Peter J Sotich (1961 – 1963)
AMA 13 John Worth (1963)
AMA 14 Maynard L Hill (1964)
AMA 16 Cliff Weirick (1967 – 1968)
AMA 17 John E Patton (1969 – 1970)
AMA 19 Earl E Witt
AMA 20 John Grigg
AMA 21 Don Lowe (1987 – 1995)
AMA 22 Dave Brown (1996 - )

4 and 5 digit numbers are either someone who's been a member for a long time OR someone who has become a Leader Member or Contest Director and chosen to get a lower number. Life members can be distinguished because their number starts with L (as in L101) and I believe they can duplicate an open number (so you can have a member with the number 1234 and another member with the number L1234). And I also believe Life member numbers start at 101.