For the function f(x) = 4sin (3x + π/2), what values will have no solutions

For the function f(x) = 4sin (3x + π/2), for what values of k wil the equation f(x) = k have no solutions?

Thanks!

sin (3x + π/2) will take any values (continuously) within [-1, 1]
so f will take values within [-4, 4]

so f(x) = k will have no solutions for any k < -4 and any k > 4