If you ask me, it's easy to tell the difference. If the word is said with real meaning, it's talking. Otherwise, it's babbling.
Speaking in sentences at 4-6 months? I don't buy it. Not only that, it's kinda creepy!
My baby started saying "Mama" with intent at 8 months. She would wail it if I walked out the room - no doubt she was meaning me. She didn't learn to say "Dada" for a long time though (she actually learned to say "reading" first
). But now she's a total daddy's girl!
She learned most of her words after the 12-month mark though, yes.
She said her first sentence at 15.5 months ("Boat is coming!"). This is considered on the early side of normal.
It's hard to generalize about talking because there's a really big span of "normal" - kind of like with walking actually. Parents can get hung up about it, but once kids grow up, you won't be able to tell the difference between the kid who started talking at 1 and the one who started talking at 2.
My daughter's an early talker but she didn't start walking till 16 months. I was the same! My brother walked at 10 months; my husband's elder daughter walked at 10 months. Go figure.