|Quite frankly I see a lot of new mamas with only one young child blasting the OP. Forgive me for stating the obvious that you are a bit inexperienced when it comes to the trials of parenthood. You need to have another kid or two and walk a few miles in the OP's shoes and then come back and tell us how great your kids get along and how well behaved they are. Helping your kids to negotiate sibling and other social relationships is a skill that is honed over a length of time and it is one that is diffficult to obtain. I have a 14 yo and a 7 yo and they still tax my patience and abilities at times.
As a mama of one young child- I agree with you! Only because I spent years as a preschool teacher, nanny, daycare worker, and got to be exposed to different behaviors in different age ranges before having one of my own. Four year olds have a pretty wide range of development, some are more like toddlers, some are more like kindergarteners (especially only children tend to be less into the hitting thing, from what I've experienced, the more siblings, the more hitting!) I agree that it needs to be made clear to the offending child that hitting is completely unacceptable- and by that token, the 18 month old should have been reprimanded in an age appropriate way as well. My DD went through one of the worst hitting/biting phases I have ever dealt with in a child (she has SN)- I was getting beaten every day on a regular basis, eyes gouged, had bruises all over, and DD is from a VERY non-violent, gentle home where she has never witnessed hitting, let alone been hit. It is very often not a reflection on parenting skills at all, but the fact that children, like all humans of all ages, have their own personalities, emotional and mental development. Shaming a child (evidenced by the head down) is never okay. Adults need to remain calm and consistent rather than explosive and threatening.