4.如图,在棱长为aaa的正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,PPP为A1D1A_{1}D_{1}A1D1的中点,QQQ为A1B1A_{1}B_{1}A1B1上任意一点,EEE,FFF为CDCDCD上两个动点,且EFEFEF长为定值,则点QQQ到平面PEFPEFPEF的距离((( )))