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I've checked through the materials too and I don't see where having four or three groups matters. With such a small class, you might consider going with three groups of four. I have forwarded your question to PILGRIMS author, Myron Flindt for further consultation and have asked him to contact you directly.
Good luck with the activity and let us know what happened in your class. We'd be happy to see some of your students' work or just a summary on how the unit worked.