Why go to 20/4 at all? Others may disagree, but the consensus seems to be that running 24 hours, non-stop, does not detract and only can help with plant growth. So, unless you believe that 20/4 is better (which some or many do) or have another good reason, e.g., save 4 hours of electricity daily, why bother with 20/4, why play games with timers?