4. 如图,在平行四边形ABCDABCDABCD中,已知AB=8AB=8AB=8,AD=5AD=5AD=5,CP→=3PD→\overrightarrow{CP}=3\overrightarrow{PD}CP=3PD,AP→⋅BP→=2\overrightarrow{AP}\cdot \overrightarrow{BP}=2AP⋅BP=2,则AB→⋅AD→\overrightarrow{AB}\cdot \overrightarrow{AD}AB⋅AD的值是 .