Specialized StumpJumper mountain bike, pristine condition, medium size +10 The Specialized Stumpjumper M4 is a line of iconic, lightweight mountain bikes produced by Specialized in the early-to-mid 2000s. The "M4" refers to Specialized's proprietary aluminum alloy, which was heavily engineered to balance a lightweight build with high durability and stiffness for cross-country (XC) and trail riding.Models spanning the M4 generation (roughly 2000 to 2006) came in several configurations that defined the mountain biking era: