That's kind of what I was getting at. I'm sure the truss' would be fine but how much weight is too much? I've come to the conclusion that truss' are made as cheap as possible and are really only made to support weight from the top (roof line). That's ideal.
I think the 4x4s would be perfect. Modification is needed. Maybe add hurricane clips also. I mean how many screeches and creeks do you hear while your standing in the rafters. Definitely strong enough.
I got the 4 post lift idea from hot rod magazines. Where they put the corvette or cobra or roadster over top of the daily driver.
I've got bad thoughts of seeing someone trying to drive the quad up and the platform falling over the direction of the quads movement. x-bracing and wedge anchors.
Imagine the speed you use to get the quad in the back of a truck. Now double the height of truck (aka platform). Now think about at that speed ducking down to miss the rafter. That's like trying to get out of the garage door before it closes and then jump to miss the obstruction sensor.

Good times right there.