A: How wonderful that you're thinking about maximizing your child's potential! There are lots of things you can -- and should -- do to boost your child's incredible development. But don't stress about having to "work hard" to accomplish this. The great thing about it is that these strategies work best when they are a natural part of your day, work for you and the whole family, and most of all -- are FUN!
Four-month-olds are just getting the hang of things -- literally. They're learning to open and close their hands, so give them various baby-safe objects to hold and move around. He'll especially benefit from face-to-face playtime with you and the rest of the family -- let him grab onto your hands, and he may start to pull on you to exercise his muscles in preparation for crawling in just a few short months.
But the "conversations" you have with your baby are the most important part of the equation. He already has an internal sense about language, and "knows" that conversations have starting and stopping points, with pauses in between for each participant to take turns "talking". Follow his rhythm in the conversation, using words, sounds, gestures, and facial expressions to communicate with him. I bet you'll start to notice that at times, he may mimic your tone or sounds. He's learning so much from these baby chat sessions! Keep them going until he starts to lose steam -- don't press it when he's tired.
That's another important point -- little babies have fairly short attention spans, and go through several "sleep/wake/activity/fussy" cycles each day. Focus your fun time with him during the "activity" phases when he's most alert and focused. And don't forget -- have fun!