Dig four holes, 12 inches deep, to form a square, with the centers of the holes 24 inches apart.
Place a 48-inch, 4-by-4-inch post in each hole. Make sure the post is plumb on each side so it is exactly vertical. Align the posts so the edges are in line with the square. Mix concrete based on the manufacturer's instructions, and fill the holes with concrete mix. Let the concrete dry thoroughly.
Drill holes through the center of each of the posts, 2 inches from the top. Make the holes large enough to accommodate 1 1/2-inch pipe. Line the holes up so a pair of posts on each side has holes that are in a direct line. To ensure the posts are directly in line, tie a string between the posts at the hole level, pull the string taut, and check the placement with a level before you drill.
Slide a piece of 32-inch pipe, threaded on each end, through each pair of holes. Screw a cap onto the threads at each end of the pipes to prevent the pipes from sliding out.