I have a 4-year old daughter. I read many child development books, and the best advice that I have found is to teach children to "use their words" when any children do anything that makes them feel bad in any way.
For example, if a friend pushes my daughter and she starts crying, I'll have my daughter tell her friend "I don't like it when you push me" in an assertive, strong voice. I will walk over with my daughter to her friend, get down to their level, and be there to remind my daughter what she could say if she chooses. Then I'll try to get her friend to respond to my daughter's comments, and get them both to talk about it.
It takes time to teach kids to be assertive, but it is so important. Not only for the ones being picked on, but to the bullies too to try to get them to think about other kids' feelings.
There is a good book called "How to be a good friend" - this is written for kids aged 4 to 8 and gives examples of all kinds of situations that kids get into with each other. It's a great conversation starter. Many times my daughter will talk about how she tried to play with some kids and they wouldn't let her join in, or some similar situation, and we always pull out that book to read and talk about things that she could try next time.
As you said, these are issues that our kids will be dealing with all of their lives!
fyi, here's the link on Amazon for the book.
https://www.amazon.com/s/ref=nb_ss_gw/104-###-###-####-##...