I'm considering using a 4"x4" for a crosspiece to put a chainfall on and
pickup the front end of tractors.

What I'd do is use scaffolding sections to get the height I need, use the
4"x4" for crosspiece, and honk up the front ends for blade changes.

About max weight would be 1000lbs of tractor. Front end would probably be
around maybe 600lb.

Will the 4"x4" hold these loads?