In most cases I agree that it would be mileage. I think sometimes the thermal cycles on a car can be worse than the mileage. If the 4 year old car was a daily driver used to commute 5 miles to work every day, then that's worse than 50k miles of mostly freeway miles in 2 years. Also a 4 year old car might have spent more time outdoors in the sun and rain.
I would rather have a car that was driven 500 times with each trip 100 miles for a total for 50,000 miles in 2 years versus a car that was driven 5000 times and driven for 5 mile each trip for a total for 25,000 miles in 4 years.