Some hybrids can experience faster brake wear on the rear, in particular Toyotas, as during the braking process the vehicle will use regenerative braking at the front wheels wherever possible to send charge back into the battery, while normal braking occurs at the rear. This means that over the course of 20,000 miles the rear brake pads will have done more work than those at the front. New brake pads have an approximate thickness of 10mm, so assuming yours have lost 4mm in 20,000 miles, they should be fine for another 10,000-15,000 miles before reaching 3-4mm when they should ideally be changed.