Sounds like a Smith's Black-Headed Snake(Tantilla hobartsmithi), a tiny burrowing species that is closely related to the Southern Crowned Snake we have here in South Carolina. What you saw was most likely an adult, not a baby, since even the adults are very small. Here's a link so you can compare it to the snake you saw: www.californiaherps.com .
No snake is "poisonous", since a poison must be either consumed or inhaled to cause harm. But no, it's not venomous, either. That is a very good description of one of my favorite snake species, the Eastern Coachwhip(Masticophis flagellum). Some adult Florida specimens are all-tan, but most adult Coachwhips have a black head and a tan body, and can reach lengths of up to eight feet, so a four-footer is still a youngster.
It would be very, very, very helpful to know some more information, please. Where was this snake seen, as in what country, what PART of the country, what sort of habitat, how large was this snake, and did it have any other observable features? Depending on where you live, "black head with a tan stripe and brown body" could describe dozens of different kinds of snakes. IF you're in the US, and this snake was very small, then it was probably a harmless Brown or DeKay's Snake(Storeria dekayii), but some Garter species or color forms could fit that pattern as well, and so could the Southern Crowned Snake(Tantilla coronata), another small, harmless species, IF you're in Florida or southern Georgia, although these usually do not have a visible stripe.
