Place one 4-by-4-inch post at a corner of the sheet of lumber, and align the two edges. Attach the post to the sheet with 4-inch nails. Hammer the nails through the lumber sheet and into the post at each corner of the post so that it is attached with four nails.
Align a second 4-by 4-inch-post at another corner of the lumber sheet, and attach it with four of the 4-inch nails. Repeat this process at the two remaining corners of the lumber sheet.
Align one of the remaining two 4-by 4-inch posts with the front edge of the bench at 1 1/2 feet across so that it is in the center of the bench. This will add extra support to the bench. Attach the post with four 4-inch nails.
Align the remaining post along the back edge of the bench at 1 1/2 feet across. Attach it with four 4-inch nails for extra support.
Paint or stain the bench the color of your choosing so that it matches your interior decor. Allow the bench to dry for 24 to 48 hours before using it.