All other things being equal, that's true. All other things are not equal though. The 4000lb+ vehicles often have worse visibility, so you have to pay more attention to make sure a kid didn't wander in front of your truck while you were waiting to make a right turn.

>A distracted driver can kill pedestrians just as easily in a 3999lb vehicle.

No they can't - a lighter vehicle has a slightly harder time killing people. It's still incredibly easy, but the pedestrian fatality rates do go down the lighter the vehicle is.