With thousands of ballots outstanding and complaints pouring in, a Montgomery County judge on Thursday granted a petition to extend by four days the deadline for returning absentee ballots. “I guess we run the risk that 17,000 people could be disenfranchised unless there’s some extension,” Senior Judge Bernard A. Moore said at a hearing in Norristown. County officials acknowledged receiving “unprecedented demand” this year for absentee ballots and said they had mailed 29,541 absentee ballots. But with the 5 p.m. Friday deadline looming, voters continued to complain they had not yet received their ballots. By Thursday afternoon, only half the ballots had been returned, while other counties were seeing return rates closer to 80 percent, officials said. “It is totally unacceptable,” said Cheryl L. Austin, a county judge and election board member who said her daughter in California was among those still waiting for her absentee ballot.