If you rode in Portland, you probably would get nowhere near that kind of mileage. I have trouble getting my year 'round rims to last that long. (I suspect the high levels of volcanic rock and dust here is the issue. Brake and rim wear exceed any other place I have lived (Massachusetts, Michigan and California) by a factor of two of more.)

4000 miles for a set of pads? They don't owe you anything!

Ben