Is that $26,000 price made up? i.e. the other guy is pretending to pay you that, but he’s not really, in order to keep tax down?
Anyway, if you can swap for $11,000 without sacrificing much/anything in the way of mileage/quality/condition it’s a no brainer. In 5 years that S4 will be worth $10,000 and the RS4 will be worth $20,000 so it’s like a $1,000 swap. Those are my invented numbers but I don’t think thye’re miles off.
Since you’re getting the car with a pseudo warranty that’s awesome, so you know everything should work. I would still find out if it’s got a compression problem…although that’s tough to quantify since RS4s of varying compression test results all act differently. At least you’ll see if one of the cylinders is fukt, but you’d kinda know that the second you drove it if it was. Would be interesting if you could wrangle a carbon clean into the deal. They probably aren’t qualified to do it.
p.s. does this reputable dealer have any other make dealers? I guess Audi isn’t one of them if so? That’s how I bought my B7 S4. They didn’t really know anything about the car and just wanted it off their lot. BMW dealer in Macon Georgia.