atan(4*x)
d --(atan(4*x)) dx
4 --------- 2 1 + 16*x
-128*x ------------ 2 / 2\ \1 + 16*x /
/ 2 \ | 64*x | 128*|-1 + ---------| | 2| \ 1 + 16*x / -------------------- 2 / 2\ \1 + 16*x /