If a, b, c, d, e, f, p, q are Arabic numerals, and b>c>d>a. The difference between the four-digit \overline{cdab} and \overline{abcd} is a four-digit number in the form of \overline{pqef}, if \overline{ef} is a perfect square number, \overline{pq} cannot be divided by 5 Divide evenly, try to find the four-digit \overline{abcd}, and briefly explain the reason.
若a，b，c，d，e，f，p，q是阿拉伯数字，且b＞c＞d＞a．四位数\overline{cdab}与\overline{abcd}之差是一个形如\overline{pqef}的四位数，若\overline{ef}是一个完全平方数，\overline{pq}不能被5整除，试求四位数\overline{abcd}，并简述理由．
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