If θ lies in the fourth quadrant, then sin(θ) < 0 and cos(θ) > 0. So we have from the Pythagorean identity,
sin²(θ) + cos²(θ) = 1 ==> cos(θ) = +√(1 - sin²(θ)) = √21/5
Then
sec(θ) = 1/cos(θ) = 5/√21
and
tan(θ) = sin(θ)/cos(θ) = (-2/5)/(√21/5) = -2/√21
