nhiltonboyfriend012111_01_X17230.jpgNicky Hilton and her beau of four years David Katzenberg have called it quits, but a source close to the couple says the two still remain good friends.

Plus, Nicky is BFFs with David's twin sister Laura! Already knowing her split with David was coming, Nicky partied with Laura earlier this week ... alongside Ashlee Simpson at Sunset Marquis.

As for why the couple split, the source tells X17online exclusively:

    "They were both traveling and working a lot. Their busy schedules left them little time to spend together. Nicky and David love and respect each other. I won't be surprised if they end up with each other down the road."


Our source adds that friends and family are hoping to convince the couple to reconcile. With their long history, we wouldn't be surprised if they end up rekindling their romance.

See Nicky hanging out with David's twin sister in Hollywood ... after the jump.