Quote from some 2003 Specialized promotion literature:

"M4 Manipulated Alloy frames. M4 is a unique aluminum alloy with great characteristics for making a bike:

Strong - 24% more than 6061 aluminum (ultimate tensile strength)

Light - excellent elongation (formability) characteristics allow tubes to be formed with thicker sections in higher stressed areas and thinner walls where stresses are less

Durable - exceptional fatigue strength means M4 will hold up to years of hard riding"

I found out elsewhere that it was a Columbus tubing product custom-made for specialized based on their Starship Series tubing. In a quote from totalbike.com, they had this to say about it:
"It contains silicon and magnesium as main components apart from aluminum, and there are also little percentages of Cu, Mn and Zn. All this means a 6000 series alloy that is very resistant to high temperatures the way few millimeters far from the welds the tube reaches its highest mechanical alloys."

The full article is here:
http://www.totalbike.com/web/article...ing_test1.html

Hope this helps.
D. "More-than-you-wanted-to-know" Minor