Tim,
Not 100% sure the Email Send program will do what you want because it is expecting a list to go out at once, not randomly or o a cadence. Cheryl can correct me here though.
There are a few options:
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Create a Program that has 4 emails and use the random number feature to try sorting people. I haven't tried this with a Cast though.
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Put the 4 emails into 4 different streams. You can use Random Number to push a lead to a random stream. Doing this might require a external random list generator and then a trigger that Says Added to List, X, Stream Y etc.
Just some ideas, try them out first.