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#37283c19-9edd-4418-abce-aee8a31d0717简单填空题动点问题立体几何

4.如图,在棱长为aa的正方体ABCDA1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}中,PPA1D1A_{1}D_{1}的中点,QQA1B1A_{1}B_{1}上任意一点,EEFFCDCD上两个动点,且EFEF长为定值,则点QQ到平面PEFPEF的距离((  ))