All children develop their own personalities and learn skills needed to socialize and play with other kids their age at different rates. Some children are very social at an early age and adapt to new people and situations very easily. Other children take a little longer to develop the skills needed to make friends and be able to feel comfortable in different situations.
At 4 years, children are beginning to develop friendships and are learning to play WITH these friends, not just next to them. If your child is having a hard time socially, talk to his teacher. See if she can pair your child with other children during the day so they can have some one-on-one playtime.
Your child may be less intimidated and feel more comfortable playing if the playtime is more structured. Also, find out if she has noticed any friendships that are starting to develop. If so, see if that mom would be interested in setting up a play date outside of school.
Once your child feels comfortable playing with other children outside of school, the friendships will probably carryover into school as well. Providing many different opportunities for your child to experience other children will help them begin to feel more comfortable and confident in making new friends.
How about trying to schedule a couple play dates outside of school? If there are any kids that your child gets along particularly well with, or kids whose social skills you think are strong, try inviting them over after nursery lets out. You can come up with some fun activities for them to do together (I'm including a link below to preschool activities), and help to facilitate their getting along. Being exposed to other kids should help your son to learn from others' social skills and further develop his own.
I think it's wonderful that you're taking proactive steps to help him socialize. Try not to worry yourself about it too much, though! Kids tend to develop and grow at different paces, and he's still very young. There is still plenty of time for him to develop socially!