If they were 4 years old (per date code) they're getting to the point where they should automatically be replaced no matter how good they look or few miles they may have. Five years is often mentioned as the max. age of ST tires. ST tires will suffer irreversible and cumulative internal damage from towing over 65 mph, towing under-inflated and/or having too little reserve load capacity or being overloaded (sometimes due to being heavier on one side from slides).
Regardless of sitting for a long period on a lot and besides the age factor, you probably have no idea how the PO treated the tires. I would most definitely have replaced them from the start. The last thing anyone wants is a "blowout" and the expensive damage it causes as you have unfortunately found out. Assuming they're ST tires, Maxxis is highly recommended by many. Some will say forget STs and upgrade to LT tires which are more durable and not have to worry about the limitations of ST tires.
