I need help with this Trig formula

The section seen by the camera is centred at 4' and the width is given by (twice, because it goes in both directions) the distance times the tangent of the angle. In your case, that would be $10 \cdot \tan(20°) \sim 3.6$ feet, which is not enough to see Frank's feet (but enough to see Frank's head). 

In order to get there, you need to put it at a distance $d$ such that $d \cdot \tan(20°) = 4$, so $d = \frac{4}{\tan{20°}} \sim 11$ feet.