4. Given the parabola $y^{2}=2 p x(p>0)$, draw two lines through the vertex $\mathrm{O}$ intersecting the parabola at points $\mathrm{A}$ and $\mathrm{B}$. If $O A \perp O B$, then the locus of the projection $\mathrm{M}$ of point $\mathrm{O}$ on the chord $\mathrm{AB}$ is $\qquad$
4. $x^{2}-2 p x+y^{2}=0(x>0)$

Analysis: It is easy to know that the line $B A$ passes through the fixed point $T(2 p, 0)$, so point $\mathrm{M}$ lies on the circle with $\mathrm{OT}$ as its diameter. Therefore, the equation of the trajectory of $\mathrm{M}$ is: $x^{2}-2 p x+y^{2}=0(x>0)$