4.
\( \overset{⇀}{a} \)、\( \overset{⇀}{b} \)、\( \overset{⇀}{c} \)是同一平面内的三个向量,其中\( \overset{⇀}{a} =(1,2)\),\( \overset{⇀}{b} =(-2,3)\),\( \overset{⇀}{c} =(-2,m)\).
\((1)\)若\( \overset{⇀}{a} ⊥( \overset{⇀}{b}+ \overset{⇀}{c} )\)求\(| \overset{⇀}{c} |\);
\((2)\)若\(k \overset{⇀}{a} + \overset{⇀}{b} \)与\(2 \overset{⇀}{a} - \overset{⇀}{b} \)共线,求\(k\)的值.