Bill Chiarchiaro , Feb 18, 2005; 11:15 a.m.
You're confusing aspect ratios, such as the 3:2 of 35mm film and APS-C sensors, or the 4:3 of compact digicams, with the archaic naming convention for sensor sizes. You're not alone in this. See dpreview's explanation of sensor sizes.
The name of the "Four Thirds" system is a reference to size, not aspect ratio.