“This is not a particularly good day for the department,” a sullen Bill Bratton said Thursday, as he dropped the hammer on four top commanders who are the targets of an escalating federal corruption probe.

The police commissioner stripped two of the men of their guns and badges and yanked the other two from their commands — a harsh penalty he followed up with a warning to the rest of his officers to keep their noses clean.

Bratton made the announcement hours after The Post exclusively revealed that Deputy Inspector James Grant of the 19th Precinct was caught accepting cash and diamonds from a shady businessman.

Once the revelation hit newsstands, Bratton called in the head of New York’s FBI office, Diego Rodriguez, to get updated on the investigation.

Grant was placed on modified duty and transferred, along with Deputy Housing Chief Michael Harrington. Brooklyn South Deputy Chief Eric Rodriguez and Deputy Chief David Colon were also transferred.

Grant was caught by the feds getting hundreds of dollars from Jeremy Reichberg, a prominent figure in Borough Park, around Christmas, sources have said.

Grant also allegedly accepted diamonds from the jeweler while driving him home from overseas trips to pick up the gems — and was told they were for his wife.

Reichberg and Jona Rechnitz, a real-estate investor, are at the center of the FBI’s probe into whether cops performed favors in exchange for lavish gifts, like trips and Super Bowl tickets.

“The potential violations under investigation include violations of NYPD rules and policies, the city conflicts-of-interest rules and the federal criminal laws,” Bratton said.

“The investigation is examining the conduct of current and former NYPD officers and several others.”

Bratton said he learned of the probe when he was appointed NYPD commissioner for a second stint in December 2013.

“This investigation — our investigation — had been under way a very short period of time,” he said.

As for why it took so long for any high-ranking cops to be placed on modified duty, the commissioner said, “All of you are very familiar with these investigations. They take time.”

“These are not easy press conferences to do,” he said. “We are taking those actions based on what we know at this time.”

Three of the four officers disciplined — Rodriguez, Colon and Grant — had received promotions from Bratton, sources said.

Bratton said he had instructed the Legal Bureau this year to educate all NYPD members ranked captain and above on conflicts of interest. Additional training will provided soon, he added.

It remained unclear where the transferred cops would land.

“They’re all going to end up in a support-type bureau or a purely administrative function within a patrol bureau,” a police official said.

More brass could face disciplinary action.

“We’re going to go wherever the investigation leads us,” said Larry Byrne, the NYPD’s commissioner of legal matters.

But he declined to comment on Philip Banks, who resigned in late 2014 as chief of department while he was being targeted in a federal corruption probe.

Harrington had worked with Banks throughout his career, sources said.

In an open letter to police officers later in the day, Bratton issued a stern warning.

“What I want you, the men and women of the department, officers and civilians alike, to know is this: The people of the city of New York depend on us and expect us to be honest, trustworthy and true — and so do I,” it read.

“We cannot effectively keep people safe if we do not live up to those expectations.”

Mayor de Blasio ducked out of a press briefing before reporters could ask about the scandal. His spokeswoman, Monica Klein, refused to answer questions about his confidence in NYPD brass.

“The mayor and Commissioner Bratton are both committed to ensuring that the NYPD maintains the integrity and trust that the public expects from its Police Department, and the NYPD is conducting a joint investigation with the FBI to discover all the facts,” Klein said.

“The mayor is fully supportive of these investigations.”