We have a raised foundation with floor joists on adjustable jacks that are 4 feet on center. We want to install floor tile but the floor flexes when walked on. What do we need to do under the house to the joist to stop the flex so the grout and tiles do not crack, etc?

Install a laminate tile.

You'll never get the deflection out of the existing floor, without major revamping and doubling of the joists and posts.