Why Is my 4-Year-Old So Mean?
“Why is my 4 year old so mean? He used to be so sweet, but my friend’s 4 year old acting out as well seemed to bring this on, because after a playdate it’s like he just decided to be mean as hell.”

You need to change the way you are thinking about your son. He is not mean, instead he is acting in a mean way. He is having trouble regulating his emotions, which is something very common for children at that age. At this age, children don’t have the ability to decide to be mean. He is not acting mean on purpose.
I encourage you to take a coach approach: Our goal as parents is to help our children to learn from their mistakes so they can do better next time. For example, if when your child hits someone, you only say “You can’t hit, that is wrong!”, you are not teaching him how he should behave. Maybe hitting is the only tool he has. Instead, give him a more effective tool. “Hitting is wrong because you hurt the other person. I see that you are angry, what could you do next time you are in the same situation? Perhaps you could tell your friend that he made you angry?”.
Praise him: as parents very often, we ignore good behaviour, and we only focus on negative behaviour. Children love for their parents to be happy with them, so the more you let them know that you like what they are doing, the more likely they are to repeat it. When you praise them, be specific. Rather than saying: “You are such a good boy”, say “Look how well you are sharing with your sister, well done!”. This way they know exactly what they are doing right and are more likely to repeat it.
And finally, and very important: model good behaviour. It seems that your son may be copying what his friend is doing. Instead, you need to be a good model and always behave in a kind and gentle manner. Children always learn more from what we do than from what we say.
Also, consider that it could also be that your child is really tired after his playdates and that could be affecting his behaviour.
I hope this information helps. Do get in touch with me if you want to discuss it further.
You may find these articles useful:
How to Discipline Your Child: An Age-by-Age Guide
Tips to Deal with a Defiant Child?
Why Is my 3-Year-Old So Aggressive?
I wish you both all the very best.
Love,
Ana