Mark M. answered 04/28/19
Retired math prof. Very extensive Precalculus tutoring experience.
If x = 4sinθ, then sinθ = x/4.
draw a right triangle with acute angle θ, label the side opposite θ as x, and label the hypotenuse as 4. By the Pythagorean Theorem, the side adjacent to θ is √(16 - x2).
So, tanθ = x / √(16 - x2).