Because she's only 4 months old. Once they sleep through the night doesn't necessarily mean they're going to continue.
at 8:06 AM on Oct. 30, 2009
Babies often start waking back up when they start the teething process. Things might be moving around under his/her gums and causing her to wake in the night. (Doesn't mean you'll see teeth soon) Plus, 4 months is kind of young based on my definition of Sleeping Through The Night (8+ hours--- for kind of sleeping through the night- 12+ (like my other kids) for the real thing).