Personally, I'd..

Not click on Gator's joke, to begin with...

In all honestly, I'd probably replace all 4 at once.. that way you know what you have.. Plus, even if tires aren't physically worn down, over age the rubber breaks down and are prone to blow outs - I think 4 years is supposedly the replacement period? I personally will push that much longer, but that's another story..

But, I'd probably replace all 4.. Check your spare - if you have a full sized spare - to see if one of your back tires is in better shape.. I do that so I rotate the tires out of the trunk, as well, even tho never really use a spare..

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Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..