And now the inner view of the bottom cap after the 3/4" threaded male PVC fitting is installed. Again, standard PVC primer and cement procedures apply here. Twist out the 3/4" male fitting, apply PVC primer to both pieces and let sit for one minute, then apply PVC cement to both pieces and put them back together. Try to only let about 1/8" of the inner fitting to protrude past the inside edge of the 4" cap so you don't create too much of a water pool in the bottom of your tower that might overflow. If you aren't sure of your work here, feel free to apply silicone anywhere you think that the joint might leak, Better safe than sorry.