You only turn 4 once -- and in honor of Kingston Rossdale's big day, his famous parents spared absolutely no expense!

A source tells that Gwen Stefani and husband Gavin Rossdale spent $15,000 for their little boy's May 29 bash at their L.A.-area home.

Where'd that dough go? The party's 100 guests -- including Kate Beckinsale and David and Victoria Beckham -- noshed on burgers, hot dogs, tacos, quesadillas, french fries and chicken fingers. In addition to the mandatory birthday cake, sweets included a cotton candy machine and a lemonade stand, the insider reports.

On hand to entertain the kids: a bouncy castle, three birds, a face painter, a balloon artist, and roaming super hero characters.

Kingston, whose actual birthday is May 26, is big brother to Zuma, 22 months. Last year, Stefani told Elle magazine that Kingston's terrible twos were a challenge.  "Kingston's whole thing is, 'I need, I need.' He is insane right now. We're just hoping for the best and that he's not going to turn out to be a freak."

The singer, 40, wed rocker Rossdale, 44, in 2002.

A rep for the couple could not be reached for comment by Us.