Ya $4k is a good price, especially if the suspension system is without fault.
Take it for a test drive and see how the truck handles on bumps and potholes. Have the seller activate whatever different suspension modes and be certain all of that is 'as it should be.'
If you look online through the differnt parts stores, you will find those shocks cost over $350 each. So, if the shocks are soft or no longer activating you could find yourself spending an extra $1000 real quick.
It's only a matter of time before it needs new shocks but better for you if they hold up for another 6-mo. Don't replace them with junk or the dummy version. It's a rip off as the dummy shock is only like $100 cheaper while built with 50-cent technology.