I would buy 4 new ones if I was you. The other two are just as old and will deteriorate like those other two very soon very likely. I just had a blowout travelling at 100km/h with my Goodyear marathons that were in excellent condition with only about 8000 miles on them. They were 8 years old however but the trailer was sitting in a seasonal lot for those 8 years. Took it out this year for a trip and bam! Blowout. From now on on my tires get changed out at 5 to 6 years wether they need or not. Tire age makes a difference in safety even if there are no visible danger signs like cracks etc.
