for 4 snakes and a clutch of babies next near, raising your own rats may be cost effective for you. It really depends on how much you typically spend on rats to feed your current snakes.
One thing I don't agree with on the setup the other poster indicates for rats, is the cage size. I can see squeezing 5 adult rats into a 20gal tank may be fine if you have 100's of snakes to feed but for a small breeding program, giving space makes the rat lives much better. Not to mention the small cages would need to be cleaned 2-3 times a week.
I use large roughneck rubbermaid containers which are roughly 4' long. A little big but this gives the rats plenty of room to move around and raise their young inside. I use 1/4" wire mesh over the corners (inside) to prevent the rats from chewing through the 'foot' indentations, about the only place they can find purchase to chew on. I just staple gun the 1/4 wire mesh on, bend the staples on the other side to prevent sharp edges and cover with a bit of ducttape.
For your group of snakes, a 1:3 group of rats should be enough. Getting them all on frozen thawed (the snakes that is) is a good idea as you can store appropriate sized rats in the freezer as needed, since it will be difficult ot have the correct size of rat available consistantly. With a full grown snake needing jumbo rats (I am guessing) and two yearlings, which probably eat rat pups and the other in between these two, you will be spending alot of time growing up your rats. A second rubbermaid container will be good to use as a weaning/grow up cage. Just feed off females before they get to be 4-5 weeks old in this cage, as then they may be bred. As the males are the ones that reach 'jumbo' size, these will be the ones you need to allow to grow for several months. If you find you are having too many babies, you can always toss your male into the jumbo grow up tank for a few months...till you start using up all your 'stores'. Or sell your extras locally for a bit of extra money.
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PHLdyPayne