Holy cow - how are you guys getting 3,000-4,000 miles out of bike tires? That's approaching motorcycle tire life! You must weigh 120 lbs. and ride on linoleum I've never gotten more than 1500 miles out of tires and that was on some Michelin carbons that I was careful to switch front/back every 300 miles or so.

Assuming they haven't met a premature demise through big cuts, I replace mine when they're squared off enough to adversely affect cornering. That's typically long before threads start to show.