With just days to go until the Iowa caucuses, Sen. Bernard Sanders of Vermont has a 4-point lead over former Secretary of State Hillary Rodham Clinton in the Hawkeye State, according to a new poll on the 2016 Democratic presidential nomination contest.

Mr. Sanders was at 49 percent in the Quinnipiac University poll released Wednesday, Mrs. Clinton was at 45 percent, and former Maryland Gov. Martin O’Malley was at 4 percent.

“Is this deja vu all over again? Who would have thunk it when the campaign began? Secretary Hillary Clinton struggling to keep up with Sen. Bernie Sanders in the final week before the Iowa caucus,” said Peter A. Brown, assistant director of the Quinnipiac University Poll.

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“It must make her think of eight years ago when her failure in Iowa cost her the presidency,” Mr. Brown said.

The numbers are similar to a Quinnipiac poll from two weeks ago, when Mr. Sanders had a 5-point, 49 percent to 44 percent, lead over Mrs. Clinton.

Men backed Mr. Sanders by a 63 percent to 32 percent margin, while women backed Mrs. Clinton by a 54 percent to 40 percent margin.

Likely caucus participants between the ages of 18 and 44 broke for Mr. Sanders by a 78 percent to 21 percent margin, while Mrs. Clinton led Mr. Sanders by a 53 percent to 39 percent margin among those between 45 and 64, and she led 71 percent to 21 percent among seniors.

Those who described themselves as “very liberal” supported Mr. Sanders by a 63 percent to 32 percent margin, while “somewhat liberal” voters backed Mrs. Clinton, 53 percent to 40 percent. “Moderate” and “conservative” voters were split, with 47 percent for Mrs. Clinton and 46 percent for Mr. Sanders.

“Perhaps more than other contests, the Iowa caucuses are all about turnout,” Mr. Brown said. “If those young, very liberal Democratic Caucus participants show up Monday and are organized, it will be a good night for Sen. Sanders. And if Sanders does win Iowa, that could keep a long-shot nomination scenario alive.”

The survey of 606 likely Democratic caucus-goers was taken from Jan. 18-24 and has a margin of error of plus or minus 4 percentage points.