f'(x) = (4 arctan(7x))'
f'(x) = 4 (arctan(7x))'
By the chain rule,
f'(x) = 4/(1 + (7x)^2) * (7x)'
f'(x) = 28/(1 + 49x^2)
and hence
f'(4) = 28/(1 + 49*16) = 28/785
In case you're not sure about the derivative of arctan: If y = arctan(x), then x = tan(y). Differentiating both sides with respect to x gives
1 = sec^2y y' = (1 + tan^2y) y' = (1 + x^2) y'
==> y' = 1/(1 + x^2)