Best to replace all 4, but if money is tight and two are still decent put the two decent ones in the front, new in the rear (at least that is what Michelin recommends and was proven to be much more stable and safe - older tires up front, new in the rear, fwd and rwd - in an article a couple months back when Motor Trend visited Michelin's proving grounds). Seems counterintuitive, but MT said it proved better when tested.