The star witness against Donald J. Trump took the stand on Monday for a fourth and final day at the former president’s criminal trial in Manhattan, fending off a fusillade of attacks from defense lawyers and acknowledging that he once stole from Mr. Trump’s company.

The witness — Michael D. Cohen, Mr. Trump’s onetime personal lawyer and longtime henchman — capped the case for the prosecution, which rested once he left the stand.

Over his week of testimony, Mr. Cohen was the only person to offer firsthand evidence directly linking Mr. Trump to the falsified records that underpin the charges against him. Mr. Trump, he said, approved a plan to fake the records to cover up a sex scandal involving a porn star.

During Monday’s cross-examination, Mr. Trump’s lead lawyer assailed Mr. Cohen’s credibility, painting him as a pathological liar obsessed with taking down the former president. But Mr. Cohen maintained his composure, while some jurors seemed to lose focus as they shifted in their chairs and their eyes wandered. When prosecutors received a second opportunity to question Mr. Cohen, they sought to blunt much of the impact of the cross-examination.

“Are you charged with any crimes in this case?” a prosecutor, Susan Hoffinger, asked him. “No, ma’am,” Mr. Cohen replied, explaining that he was there merely as a “subpoenaed witness.”