By ELVA RAMIREZ, KATHLEEN LUCADAMO and ROBERT F. MOORE

Additional reporting by Jonathan Lemire and Shirley Wong

The sad letter foreshadowing the disturbing deaths of a Manhattan family of four was written by the children's mother before she executed a murder-suicide plot with her husband, a police source said yesterday.

Christine Wang, 41, sent the letter by express mail to her mother in Taiwan, detailing the business failures of her husband, Fred Wang, 42.

"The letter sounds like she was saying goodbye," the police source said.

Stunned by the letter, Christine Wang's panicked mother called Ping Wu, 50, of Paramus, N.J., who is Wang's cousin, and asked her to check on the family, authorities said.

Wu raced to the Wangs' second-floor apartment on W. 123rd St. in Morningside Heights on Wednesday. But Fred Wang's 86-year-old grandmother, Hwei-Mink Wang, told her they weren't at home.

Wu left a phone number with the grandmother, whom another police source described as in the early stages of senility.

When Wu didn't hear from the family, she returned to the apartment Thursday. She walked into a bedroom and found the couple and their kids, Serena, 6, and Dennis, 8, dead.

Fred Wang was lying on a mattress. His wife was found on another bed next to the children.

Wu called cops and firefighters, who arrived just before 8 p.m. to find charcoal spread among six pots and pans. Carbon monoxide levels in the room measured 71 parts per million, well above the 10 parts per million considered safe.

The bedroom windows were closed and a rug blocked the space below the door, which may have saved the grandmother from a lethal dose of carbon monoxide. The elderly woman was taken to St. Luke's Hospital, where she was in stable condition last night, police said.

The medical examiner's office said it wasn't clear how long the family had been dead, but cops believe it was at least two days.

Officials at St. Hilda's & St. Hugh's Episcopal School, where Fred Wang paid $50,000 a year to educate his kids, said they had no idea about his financial woes. He made timely payments to the prep school on W. 114th St.

But the letter sent to Taiwan said Wang's New Jersey-based business, FCW Trading Inc., had not been active since 1998. It revealed the 1985 Columbia University graduate had lost money on an overseas deal involving livestock and grain, a police source said.

Neighbors said the family's finances led Wang to sell his apartment and move into his grandmother's home in the same building about a year ago. The family was considering sending Serena and Dennis to public school.

"They were great kids," said Virginia Connor, who heads the Episcopal school. "We adored these children."