Cut the 4x4 into four 11-1/2"-long pieces. The back of the 4x4 will be notched out so that it wraps the frame corner, is flush at the bottom and protrudes 1 1/2" above the top of the frame. Start by marking the cut-out with a 10" line 2" in on two adjacent sides. Set your saw blade to a 2" depth and make these two 10"-long cuts (cuts 1 and 2).
Next, make the two perpendicular cuts with the same saw depth (10" from the end, cuts 3 and 4). There will now only be a small piece of wood connecting the cut-out corner and the drop. Using a vibratory saw or a chisel, remove the final wood and clean up the inside corner. Lastly, cut the back of the 1 1/2" top at a 45-degree angle. Glue the interior faces of the corner and finish nail to the bed frame.