How many feeders should I buy

I am going to a reptile show on saturday and trying to figure out how many rats and what size I need for 4 weeks worth of feeding. I have 4 snakes that only eat live. So I need 4 live small rats every week. So a total of 16 rats. The problem i run in to is they grow fairly quick so in order to get 4 small adult rats every week I need to buy some small and large hoppers to be able to have 4 small by the forth week.

I was figuring:
week 0: 4 small rat (right away feed)
week 1: 4 large weanlings
week 2: 4 large weanlings
week 3: 4 small weanlings
week 4: another show restock

So by week 3 the small hoppers would be small adult rats. Is this a good plan or do you think I should mod it some what.