4ply vs 6ply

Whentruck was purchased in 2000 it had 4ply tires  I switched to 6 ply because it was a work truck.  Now that I'm retired I wanted to go back to 4 ply but my tire store says I can't go back unless I change all four.

I want to change only the front two today & next time the rear two.  Is this a law?

Check with your local motor vehicle department and if not (the law), find a different tire store.

I would say it's okay, just don't go below the manufacturers suggested ply rating, (light duty, medium,heavy duty).

It would be best to change them all at the same time, but it is not the law.  They just want to get some more money out of you.  As long as you keep the two like tires on the same axle it will be fine.