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. http://www.babycenter.com/0_20-fun-silly-development-boosting-games-to-play-with-your-ba_1479310.bc.
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.