Does this mean that you're not familiar with the meanings of the inverse trigonometric functions?

In case you're exaggerating a bit...

By definition, arctan(4) = A means tan(A) = 4 = 4/1.

Draw a right triangle. Label the non-right base angle as A. Label the "opposite" and "adjacent" sides as 4 and 1, respectively.

Use the Pythagorean Theorem to find the length of the hypotenuse.

Read the value of the sine from the triangle.