For the differential equation $$s'' + b s' + 4 s = 0,$$ find the values of $b$ that make the general solution overdamped, underdamped, or critically damped.

(For each, give an interval or intervals for b for which the equation is as indicated. Thus if the the equation is overdamped for all b in the range $-1<b\le 1$ and $3\le b <\infty$, enter (-1,1], [3,infinity); if it is overdamped only for $b=3$, enter [3,3].)

If the equation is overdamped, $b \in$
If the equation is underdamped, $b \in$
If the equation is critically damped, $b \in$