I only have 3 snakes; a baby corn and 2 baby balls. I breed my own mice and ASF's and theres np at all doing so! It will sive you money and you can make money out of it. Persanally I dont think you need to build a rack to feed only 4 snakes but, it is fun to do and also a learning experince. (was for me!) If I were you I would just run two 1.2 colonies in 10gal tanks. In 3weeks time you should have 32pinks and in another 3weeks you should have about 32 ready to wean pups. (That is just based on my litter survivail from four mothers) If you do go woth building a rack wich there is nothing wrong in doing so I would go with the 16x12x4 tubs. You could prob run a 1.3 mabye a 1.4 but, that could be a little stuffy for the mice. I use a 18.5x15.5x4 tub and I can run a 1.4 np. Everyone has room to nurse thier babies.
Hope I helped and good luck breeding!