Nicky Hilton, David Katzenberg reportedly split

Nicky Hilton and her boyfriend of four years David Katzenberg have reportedly ended their relationship. The split was amicable and caused by their demanding work schedules.

A friend of the couple tells New York Post's Page Six, "The main reason was that they have been busy with their work schedules, and it has made it difficult to spend time together."

Hilton has recently been traveling overseas to promote her jewelry line while Katzenberg (the creator of MTV's "The Hard Times of RJ Berger") has been busy starting a new production company. A friend says, "Their schedules never seemed to match."

The source says Hilton, 27, and Katzenberg, 28, will remain friends despite the break up. "They have been friends since they were teenagers."