Have heard of sibling jealousy but my 4 yr old boy is sometimes jealous of mommy kissing dad in front of him...how do you assure the kid?

I've read about this phenomenon when I took a child psychology module in university. I'm no expert on this matter but apparently boys around the ages of 3-6 often experience a phase called the "Oedipal Complex" in which they "love" their mother and are jealous of the father who is seen by them as a rival for their mother's affection. This behavioral phase of a boy’s psychological development was named by Sigmund Freud after the Greek mythological figure, Oedipus Rex. Oedipus unknowingly fell in love with his mother and married her after he killed her husband who, of course, was his father. That is of course just a myth and nothing dramatic will happen in your case. Just keep reassuring your son that there is plenty of love to go around, be firmly gentle as your son plays out this phase. Don't worry too much and don't over-react. If you do not treat it as a problem it will not become one. Like other developmental phases, it will pass :)