Current culture system. Top off reservoir

Whats up fellas,
Recently purchased a uc4xl (4 buckets). First time with this unit. I'm trying to setup the top off reservoir now. I don't have any problem setting it up my question is really based on maintaining it. While reading up on the current culture website it says the best thing to do is have your next week's nutrients in the top off reservoir. So if your in week 3 flowering you would put week 3 nutrients into your main system and then week 4 nutrients into the top off reservoir. So how exactly does this work? So during that week 3 of flowering whatever is used in the main system will be replaced from the top off reservoir. At the end of that week if the top off reservoir is down say 3 or 4 gallons (just an example because I've never ran this unit) what exactly am I supposed to do to get week 5 nutrients into it?Do I need to drain the top off reservoir and replace it weekly or do I fill whatever amount has been used with week 5 nutrients?

Appreciate any help regarding that question and any tips moving foward from anyone with experience with these systems.

Thanks