At the end of the third grade rhythm unit, we learn that beats are usually grouped in 3's or 4's. We sing songs in both 3 and 4, listen and find the strong beats, and "ta" rhythms in both meters.
It can be difficult teaching students how to hear the difference between meter in 3 and meter in 4. I have found it to be easier for students to hear the difference between 3 and 4 than between 2 and 3 (or duple and triple).