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\(∆AED,∆CFD,∆BEF \)分别沿\(DE\),\(DF\),\(EF\)折起,使点\(A\),\(B\),\(C\)重合于点\(P\),如图\(2\)所示。
\((I)\)求证:\(GR⊥ \)平面\(PEF\);
\((II)\)若正方形\(ABCD\)的边长为\(4\),求三棱锥\(P-DEF\)的内切球的半径。