I am assuming you mean 4/7 as the sine or cosine cannot be higher than 1.
Lets find θ,
θ = [sin-1](4/7) = 34.85 °
But lets take into account that this value is the equivalent in Quadrant I.
If Θ lies in Quadrant II , then θ = 180 ° - 34.85 ° = 145.15 °
So cosθ = cos (145.15) = -0.821
