My baby babbles a lot! Is this normal? She's just 4 months old and can "talk" for a ten - 15 mins at a stretch, multiple times a day.

You should be happy, cause you don't need to worry that when they should learn to speak that period and their mouth is close tight.. Do talk to her in proper language instead of baby language, in fact, one day she may sound it similar when she is suddenly babbling. My son say pororo long ago b4 he was one, just that whenever he say that, i didn't capture in video, and do note, it's not papa mama, but pororo, and it's really when he saw pororo, he say pororo.