Do I get new tires and how many?

I have a 2006 Honda Civic with three tires I feel need to be replaced, but one that is still good on tread. The readings are 3/32, 3/32, 4/32, and 7/32. The less worn tire is newer, and by the manufacture date, appears to have been put on the vehicle right before I purchased the car used in 2013. The three lowest... show more I have a 2006 Honda Civic with three tires I feel need to be replaced, but one that is still good on tread. The readings are 3/32, 3/32, 4/32, and 7/32. The less worn tire is newer, and by the manufacture date, appears to have been put on the vehicle right before I purchased the car used in 2013. The three lowest ones all have a leak that costs ~10 psi a week so I'm constantly having to fill them, and they are the worst tire I have ever used, thus I want to replace them. Is it recommended to replace all four tires to match, or just the three bad ones?

Also, if I were to consider selling the vehicle soon, should I even bother to replace the tires? Will it affect how much I can get for the vehicle if it has fresh tires rather than old tires? I don't exactly want to spend $500 on new tires if the vehicle can be sold for the same with or without new tires.
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