Unscrew the old toilet flange and pry it loose. Remove the sub-floor in order to expose about 2 feet of the 4-inch sewer pipe. Cut the pipe about 16 inches from the shower drain location.
Join the 4-inch to 2-inch PVC reducer bushing to the sewer pipe with a 4-inch rubber coupling, if the pipe is metal or non-PVC. (The coupling comes with a pipe clamp that you must screw tightly over the existing pipe.) Attach a 4-inch to 2-inch PVC reducer bushing to the sewer pipe with PVC primer and cement, if the pipe is PVC.
Temporarily mount a 2-inch PVC trap under the shower drain location with a pipe hanger. Cut a length of 2-inch PVC pipe long enough to connect the trap to the reducer bushing. Slope the 2-inch pipe toward the reducer bushing, allowing it to drop at least ¾ inches in elevation for every 1 foot of length. Mark the location of the 2-inch pipe on the floor joists with a pencil. Remove the trap and pipe.
Attach the 2-inch pipe to the adapter with PVC primer and cement. Mount the 2-inch pipe to joists or flooring with pipe hangers, using hammer and nails.
Attach the 2-inch PVC trap to the 2-inch pipe with PVC primer and cement. Measure the distance from the top of the trap to the planned surface of the shower, and subtract the height of the shower drain assembly. Use that length to measure and cut a piece of 2-inch PVC pipe. Attach the 2-inch pipe to the top of the trap with PVC primer and cement.
Unscrew the top half of the shower drain assembly and set it aside. Attach the bottom piece of the shower drain assembly to the 2-inch pipe with PVC primer and cement. Lay the sub-floor. Continue with shower construction.