Q: it seems like humans tend to organize numbers in 10s because we have 10 fingers. any thoughts why rhythmic beats are usually grouped in 4s?
A: yes, i think the important number is actually 2. multiply it by itself a few times and you will see all those standard beat and bar quantities  (2, 4, 8, 16, etc). Besides powers of 2, the next number that seems to play by far the most significant role in rhythms, melodies, (and our minds in general!) is 3.  Also i should mention, another number that plays a huge role particularly in songwriting, and story telling in general, and is often overlooked, is the number 1