So-called BFF JOHN TRAVOLTA neglects to attend star-studded send-off of late SOPRANOS star. WHY???

MORE than four years after the tragic death of his teenage son, John Travolta is still too overwhelmed by grief to attend a funeral.

That’s the REAL reason the “Pulp Fiction” star was a no-show at his close pal James Gandolfini’s celebrity-packed memorial at The Cathedral Church of Saint John the Divine in New York City on June 27.

“He’s still despondent over Jett’s death, and the last thing he wants to do is see a casket,” said a source.

Jett Travolta was just 16 when he died in January 2009 after suffering a seizure at the family’s vacation home in The Bahamas.

While friends and family were paying their last respects to the “Sopranos” star, who died of a heart attack in an Italian hotel room on June 19, John, 59, was photographed with soccer star David Beckham in London, where he was promoting pricey Breitling watches.

But two days earlier, on June 25, the actor paid tribute to Gandolfini at a British Film Institute event in Travolta’s honor.

As NationalEnquirer.com reported,  he vowed to look after the family of his late friend. Gandolfini is survived by his second wife, Deborah, and their infant daughter, Liliana. He also has a 14-year-old son, Michael, from his first marriage.

John told a reporter: “My goal is to make sure his family is OK…We’ll just make sure they’re taken care of.”

The actors hail from the same area of New Jersey and had worked together in the films “Lonely Hearts,” “Get Shorty” and “The Taking of Pelham 123.”

Although John missed the funeral, he called Deborah to offer his condolences.

“John’s willing to do anything he can for the sake of Jim’s kids,” said the source.