Just nine months after their R&B-lite debut made the bubblegum set scream, the teenage quartet already have a second disc. This time, between sweet-harmonied ballads, they cue up randy singles (''Bump, Bump, Bump'' with P. Diddy), bouncy odes to cruising chicks (''Back It Up''), and ''gangsta'' boasting. Shouldn't someone be setting a curfew?

