Some schools which allow students to take calculus 1 and 2 before senior year also offer multivariate calculus (3), differential equations, and linear algebra courses to round out the fourth year of math. This is especially prevalent when students can take geometry in 8th grade which leads to Algebra II, Pre-Cal, Calculus, Advanced Math Electives as the four year progression.

My public high school in TX had that track.

Is this uncommon in California schools?

(I think the idea that it's in any means required to be able to do STEM in college is ludicrous, but having the option is great.)

My public high school in WA also had this. "AP Calculus" covered Calc I and II, and once you had completed that there was a special elective called "Advanced Calculus" that covered Calc III (and prepared you for the AP Calc BC exam). In my graduating class about 30/430 ended up in this Advanced Calculus class senior year having finished AP Calculus as juniors, but many more who completed "regular" AP Calculus as seniors also ended up in STEM fields.

I took Advanced Calc and AP Stats senior year, which in retrospect was a mistake :P