如图\(1\),在正方形\(ABCD\)中,点,\(E\),\(F\)分别是\(AB\),\(BC\)的中点,\(BD\)与\(EF\)交于点\(H\),点\(G\),\(R\)分别在线段\(DH\),\(HB\)上,且\( \dfrac {DG}{GH}= \dfrac {BR}{RH}.\)将\(\triangle AED\),\(\triangle CFD\),\(\triangle BEF\)分别沿\(DE\),\(DF\),\(EF\)折起,使点\(A\),\(B\),\(C\)重合于点\(P\),如图\(2\)所示.
\((1)\)求证:\(GR⊥\)平面\(PEF\);
\((2)\)若正方形\(ABCD\)的边长为\(4\),求三棱锥\(P-DEF\)的内切球的半径.