4-Months

According to BabyCenter, here is what's going on with RJ at 4-months:

Researchers believe that your baby can now understand all the basic souunds that make up his native language. Between this time and 6 months of age, he'll develop the ability to make some of these sounds, which means you may hear the words you've been dreaming about, namely "mama" and "dada." While child development experts say it's too early for your baby to connect those sounds with you, that won't make hearing them any less exciting.
You can encoiurage your baby's attempts at coomunication by mirroring or imitating his expressions and sounds. He may try to imitate uou too. Say "ba" and he may try to say it back.
Reacting when your baby makes noises or tries to say something will help him learn the importance of language and better understand cause and effect. It's great for his self-esteem too: He'll begin to realize that what he says makes a difference.