[JRTJH]

You say, "The tires are 4 years old, two just sitting and two in use"...

I take it you mean that the trailer sat on the dealer's lot for 2 years before you bought it ??? If so, those are two years of "questionable tire care"... I've walked through dealership lots where every trailer had visibly low/flat tires and I know those trailers are moved around the lot regularly, either to get a trailer out that was sold or to make it appear trailers are moving from the dealership as well as towing them across town or across the county to an RV show...

So, those two years may have caused irreparable damage to the tires, even before you ever laid eyes on it. That doesn't include transportation from the Keystone factory to your dealer's lot.... There's no telling how fast the delivery driver towed the trailer, whether he checked tire pressure along the route, whether he "hit the curb on every corner" or whether he was "super careful and the tires were never damaged during deilvery"....

There's a "boatload" of things that could go wrong with tires while sitting on a dealer's lot for 2 years.....

If it were me and I was "unsure" of my tire condition, I'd opt for a "go from the unknown to the known" and can the unknown tires, opting for 7 new ones (yes, replace the spare as well) so that going forward, you'll know EXACTLY how your tires have been cared for and what they have been exposed to along the way.....

As for tires, to me, like with everything else, I'd stick with a "known brand"... Sailun has earned an excellent reputation among heavy RV'ers. As for the Hercules brand, from the little I've heard or read, there's no "warm fuzzy feeling" associated with the brand..... YMMV