Stanza 4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

and should some why completely weep
my father's fingers brought her sleep:

Now we hear about how the father was not only good at inspiring people, but was good at comforting people as well.
Interestingly, the woman who's weeping in these lines is described as a "why."
Here again, we see Cummings using words in unique ways. Describing a person as a "why" makes us think of a person who has a lot of questions. Maybe her questions are so deep and troubling that they define who she is.
Never fear, Ms. Why, the speaker's father is here to bring you comfort and let you sleep for a while.
Notice how in the previous stanza father woke people up, but now we learn he knocked people out (in a good way) when it was needed as well.

vainly no smallest voice might cry
for he could feel the mountains grow.

In these lines, it seems like Ms. Why is crying because she feels so small.
Maybe her questions about the universe make her feel unimportant or meaningless sometimes. It seems like some of the comfort that the father brings comes from the fact that he's in touch with the bigger picture somehow.
Anybody who can "feel the mountains grow" has got to be hip to something pretty awesome, right?