If the calculator is already in radian mode, you just have to take the arcsin of 0.4 and it will provide the answer.

The arcsin function is usually right above the sin function on your calculator. You just have to hit the [2nd] button (usually upper left corner and yellow) and then just hit the sin button

If your calculator is in degrees mode, you will have to multiply the angle you get by Pi/180 to convert it to radians.

Hope this helps.