4 month old male falls (3.5 ft) out of mother's arms onto floor hitting head. Cries immediately, small frontal hematoma, acting normal now. Head CT?

According to Kuppermann et al's recent study published in the Lancet, YES because of severe injury mechanism.

In abbreviated form:

For those metric challenged, 1.5 m = 4.92 ft and 0.9 m = 2.95 ft.


Source

Kupperman et al. "Identification of children at very low risk of clinically-important brain injuries after head trauma: a prospective cohort study" Lancet. Oct 2009.

Image source: http://www.medscape.com/viewarticle/711739