I have a four month old and I wonder how much he would get out of being around other infants and children. I realize that infants do not actually play together, but I feel like viewing other children playing could be important for development. Do early interactions lead to better relationships with peers later on?
At 4 months they're not going to get much out of parallel play yet.
But there is an important thing YOU can get out of it... talk time with another adult. There's a reason they're called "play*dates*", it's kinda like dating when you were a teen... part of it is to get to know the other parents to know if over time these are parents you're ok with your child being over at their house playing with their kids in a few years when they're older. It can also serve as the only chance to have a normal human conversation during the day.
This article from BabyCenter says that babies under 12 months will be interested in other babies and view them as toys, rather than actually play with them (no mention on if this is good for development).
Children between ages 1 and 2 will play next to another child--called "parallel play"--without really playing with the other child. Not until your child is 2 or 3 will he be able to really play and interact with other children.
That said, I suggest that you definitely set up some play dates. They'll be fun for you, and your baby can play with a new "toy" ;). The article does note that babies can easily injure other babies because they don't know not to pull hair or grab faces, so you may have to watch carefully.