While every child develops differently, typically by the age of four your child is really into playing with other children now.
This is when they really like to play make-believe, they really have strong interests and they'll tell you about them, and they like to cooperate with other children. So this is really a fun time that you can encourage opportunities of playing with other children. You can create the environment where they're playing make-believe using doctor set, using a kitchen, a play kitchen, using other opportunities like dress-up, make-believe, trains, dolls, Barbies, anything that will get them engaged in make-believe play with other children is great.
This is also an age where they'll love to tell you stories and they'll like to tell you the ending of stories that they know so encouraging more reading of books with them, playing games with them, anything that will encourage playing with other children, and getting them to work together is fun too because your child again will cooperate with other children and want to play with them at this age. This is really a fun time when a lot of a language is being used and there's a lot of more exploration too with make-believe play and they'll really want to be talking to you and telling you all about the fun things that they learned and that they do.
So have fun and remember that the more opportunities they have, the better of they'll be.