4.如图,在长方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,AA1=AB=2AA_{1}=AB=2AA1=AB=2,BC=1BC=1BC=1,点PPP在侧面A1ABB1A_{1}ABB_{1}A1ABB1上.若点PPP到直线AA1AA_{1}AA1和CDCDCD的距离相等,则A1PA_{1}PA1P的最小值是((( )))