4 The equation of a curve is \(y = x \tan ^ { - 1 } \left( \frac { 1 } { 2 } x \right)\).
- Find \(\frac { \mathrm { d } y } { \mathrm {~d} x }\).
- The tangent to the curve at the point where \(x = 2\) meets the \(y\)-axis at the point with coordinates \(( 0 , p )\).
Find \(p\).