It's unusual to have a 4-year-old in kindergarten. Is your son attending a private school, not public? If so, then it's important to realize the guidelines for behavior can be established by the individual private school, and they might be different than one might otherwise see.
Suspension for pushing another child sounds harsh, but if the other child was injured in the incident, or if your child had been warned before about staying away from the other child, and not pushing, yet he did it again anyway, it could very well be within the school's guidelines for disciplinary action.
How long is your son suspended for? If it's just "for the rest of today" then the school may simply be looking at it as a kind of "time out" and not a severe punishment. Have the teachers spoken to you in the past, about your son's behaviors? What explanation does your son give, for why he pushed the other child?