4.
已知抛物线\(E\):\(y^{2}=2px(p > 0)\),斜率为\(k\)且过点\(M(3,0)\)的直线\(l\)与\(E\)交于\(A\),\(B\)两点,且\( \overrightarrow{OA}\cdot \overrightarrow{OB}+3=0\),其中\(O\)为坐标原点.
\((1)\)求抛物线\(E\)的方程;
\((2)\)设点\(N(-3,0)\),记直线\(AN\),\(BN\)的斜率分别为\(k_{1}\),\(k_{2}\),证明:\( \dfrac {1}{k_{1}^{2}}+ \dfrac {1}{k_{2}^{2}}- \dfrac {2}{k^{2}}\)为定值.