The best footing you can make is to dig a 16" deep hole, depending if the ground is wet or not under there fill 6" with dry or wet cement, (dry if the ground is wet, you get a harder footing if it soaks in the moisture from the ground) add your 4x4 tight into place and fill the rest up with wet or dry concrete. Sub floors are my specialty. I am a flooring contractor for over 20 years now so i am not just giving you wannabe info.
