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.

yes definitely normal! it's great that she is so interactive! it helps to speak more to her, like telling her what you're doing when you're changing her diapers, telling her what she can see while pointing it out to her etc. she will develop faster in terms of speech when she grows older!