4 month-old's development.

Highlights
Sitting Pretty
Ba ba ba ba ba ba
Peaceful Goodnights
Remember your baby is an individual

Sitting pretty
By now, your baby's physical developments are coming fast and furious. When lying on her back, she'll lift her head and shoulders as you reach to pull her up. If you place her on her stomach, she'll extend her arms and legs and arch her back. This is good exercise for strengthening her neck muscles, and it will help her develop the head control necessary for sitting up.

When you pulled your baby into a sitting position a few weeks back, her head lagged behind her arms and shoulders. Now, she can anticipate the direction you're pulling her, and her head will follow right along with the rest of her body. What's more, her spine is straightening, allowing her to sit unsupported, which frees her hands for exploring, and she can pivot to reach a desired object.

Ba ba ba ba ba ba

Your baby is adding new sounds to her language repertoire, and she may sound like a broken record. Babies this age often become so enthralled by a newfound ability that they get stuck on it for a while. This is normal — babies tend to master one skill before moving to the next.

Hearing the same sound over and over can get annoying, but practicing patience now will prepare you for the endless string of "nos" and "whys" you'll be hearing from your child soon enough.

Your baby may also watch your mouth intently when you speak and try to imitate inflections and utter consonant sounds such as "m" and "b."

Peaceful Goodnights

Following a well-established nighttime routine, which might include a bath or a bedtime story, for example, may help settle your baby to sleep. It's a good idea to consistently "sequence" your baby toward slumber: Feed her, give her a bath, put her in her pajamas, play a game, read her a book, sing songs or play some music, then put her down. A beloved routine gives you and your baby plenty of time to connect and wind down. You may decide to alternate going-to-bed activities with your partner (you do the bath, he reads the story, and so on). Or, to give both of you a regular break, try taking turns, with one person responsible for putting your baby to bed each night.

Remember, your baby is an individual

All babies are unique and meet milestones at their own pace. Developmental guidelines simply show what your baby has the potential to accomplish — if not right now, then soon. If your baby was premature, keep in mind that kids born early usually need a bit more time to meet their milestones. If you have any questions at all about your baby's development, ask your healthcare provider.

*source are from http://www.babycenter.com