How can I stimulate my four to six month old child during playtime?

Play. This is a nice little article with lots of developmentally appropriate things to do. Both play, and other things to stimulate your baby.
Couple suggestions. When awake, your baby may enjoy watching you work. Baby may scoot or roll so a playpen or safe floor area is a must. Floor time helps baby build strength it will need to crawl at nine months. Cuddly toys or blankets are favored at this age. A rattle or other toy may be held and released at this stage.