seems like you have a pretty decent setup the only thing i would suggest is not leaving the fan on 24/7 nor putting it directly on your plants as it tends to make the buds very "airy" and light i would just leave your fan on while your lights are on to be honest and as for using a 20/4 light sched. there has been alot of studies showing that anything after 18hours really doesn't affect your plant and your just wasting electricity 16/8 or 18/6 sched is the most cost efficient , but if you already have it on 20/4 i wouldn't change it now as its probably to late and could possibly induce flowering but i doubt it good luck and nice setup