4. 在平行六面体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,点PPP是线段BDBDBD上的一点,且PD=3PBPD=3PBPD=3PB,设A1A→=a→\overrightarrow{{A_1}A}=\overrightarrow{a}A1A=a,A1B1→=b→\overrightarrow{{A_1}{B_1}}=\overrightarrow{b}A1B1=b,A1D1→=c→\overrightarrow{{A_1}{D_1}}=\overrightarrow{c}A1D1=c,则PC1→=(\overrightarrow{P{C_1}}=(PC1=( )))