The best song on Rilo Kiley’s fourth studio album, “Under the Blacklight,” is a simmering plateful of Southern soul called “15.” It’s a delightfully crass little yarn with a stately horn arrangement, about a lonesome teenager and the “spider on the Web” who coaxes her from her keyboard. It’s not exactly what you’d call “classic” Rilo Kiley, since the foursome from the moneyed slums of Los Angeles only started doing its best “Dusty in Memphis” impression on 2005’s slickly produced “More Adventurous.”But the song sounds instantly inviting, a logical progression after three albums of increasingly sophisticated pop music, and a window into the group’s keen talent for reinvention.

No, wait a minute.

The best song on Rilo Kiley’s new album is the danceable ode to miscommunication “Breakin’ Up,” a soaring slice of disco-pop that takes off with sugary new-wave synthesizers and lands with a bounding refrain that could have been ripped from a Donna Summer record. It’s a striking departure from the continuously maturing guitar rock and blue-eyed soul that Rilo Kiley has cobbled into a signature style since its 2001 debut. But it sounds no less assured, and with the band’s cushy new major-label deal with Warner Bros., it could be the radio smash that vaults the group into the stratosphere.

So which is it? Or maybe the better question is: Which is the real Rilo Kiley? With most bands, this kind of musical schizophrenia leaves the overall package undercooked. From faux string sections to mandolins, from electronic beats to hand claps, the sheer breadth of stylistic ingredients on “Under the Blacklight” runs the risk of alienating listeners. The Latin-rooted “Dejalo” is a Bacardi Breezer of a song that attempts to bridge the gap between Debbie Harry at her most talkative and Miami Sound Machine’s patently ’80s tackiness. Elsewhere, “Give a Little Love” skips along on a processed hip-hop cadence, while the country rocker “The Angels Hung Around” includes a cameo from the distinctly un-hip-hop Jackson Browne.

No, the alchemy doesn’t always work (“Dejalo” doesn’t get any better no matter how many times I hear it), but for the most part it all seems to gel for these Californians, who have infused “Under the Blacklight” with a carefree spirit of exploration and a simple willingness to make the music they want to make without apology. Despite the band’s increasingly radio-friendly palette and its pasting of incongruous styles, it seems singer Jenny Lewis, guitarist Blake Sennett, bassist Pierre de Reeder, and drummer Jason Boesel are in no hurry to impress anyone but themselves.

The glue, as always, is Ms. Lewis, and it’s impossible to overstate her importance to the band. She’s one of very few singers on the rock scene, female or otherwise, whose voice — a salacious blend of purity and potty mouth, poise and vulnerability — is a renowned musical instrument. Ever since she first opened her mouth with Rilo Kiley six years ago, she’s had the boys in the room batting their eyelashes and the girls flexing their muscles. A year ago, she was doing her best Loretta Lynn while touring in support of her gorgeous, shambling solo record, “Rabbit Fur Coat,” and few doubted that the return of Rilo Kiley was only as certain she thought it was. And that’s not just because she’s the sexy valley girl standing center stage (though she is that, too): Ms. Lewis’s songwriting (undertaken with Mr. Sennett), and especially her lyrics, affords Rilo Kiley its personality, juxtaposing charming melodies with explicit confessions and pert indignation. One of the band’s trademarks is a persistent contrast between the brightness of the music and the darkness of the words.

On “Under the Blacklight,” much of the music has gotten brighter and many of the lyrics darker. Rilo Kiley is a quintessential Los Angeles band, and an undeniable bond to the heavy Southern California soul of Fleetwood Mac pervades the record, fusing tales of dead-end prostitutes and chain-smoking nymphettes to Mr. de Reeder’s slinky bass lines and Mr. Sennett’s resounding guitars. On the lead single, “Moneymaker,” a wiry song about a lady of the night grappling with second thoughts, Ms. Lewis sings, “You’ve got the moneymaker / They showed the money to you / You showed them what you can do / Showed them your money / now you get out, out, out.” Of course, the song could just as easily be about a singer with a new major-label deal, but anyone who grew up in the porn mecca they call the San Fernando Valley can’t be blamed for seeing the virtue in the metaphor.

Lots of people are being used on ‘Under the Blacklight,’ but even if shouts of ‘sellout’ follow the record to the radio, Rilo Kiley isn’t among them. It’s a band in transition, unwilling or unable to settle on a defining sound — and why should it? Whether “15” or “Breakin’ Up” is the best song here is less important than knowing that this excellent album won’t be the one Rilo Kiley is remembered for. And that’s a good thing.