That is neat you have four and another on the way. We have seven, 2 girls & 5 boys, 20 yrs to 5 yrs.
Two books came to mind for resource: The Mind of Boys by Michael Gurian & Kathy Stevens and The Wounded Spirit by Frank Peretti deals with bullying and has a great resource list including for those coming out of homosexuality. The author was teased terribly in school growing up as he had a disfiguring medical situation. The first book deals more with why education is painful experience for many boys with practical solutions for parents and teachers.
You are welcomed to email me and I can give you our phone number as well if you would like to talk. Also, I have a book list of resources I would be happy to mail you. I am not saying one does not know what they are doing but the more informed you are, the more armed you are especially as a wife and mother.
I was made fun of badly as a child because of ears that stand away fm the head more (later parents had plastic surgery to pull them back). It was painful to be made fun of. Later, in tenth grade while in band class, God showed me I would travel and see things that my peers would not. From that moment on, I did not care a hoot about not being in the 'in group'. Many are still in the same pokey hometown.
We homeschool so the bullying and peer pressure is not an issue but we do not hide them from reality either. They are around all kinds of kids from church, sports and Civil Air Patrol. Homeschooling does not guarantee a non rebellious child (good marriages does that) but it helps with that and helps them not care so much about the peers esp. when they see they are way ahead in real life and not birthing babies unmarried with no job skills, not seen a bit of life and travel first. Self preservation is a real motivator.
I told a nephew one time 'your peers are not going to pay your taxes, insurance, put a roof over your head or be there for you when the boulders in life come along only your family and friends that you have developed a real relationship with'. I have heard from one high school mate one time in a short email and still in touch with my high school best friend (although a grade lower; she and our oldest have emailed as well which is neat) and that is it from school all these years.
Hope this helps.
P.S. After reading other responses that reminded me all of kids know boxing, grappling and trigger points. They box and work out regularly.
One of my read aloud books one years was Tough Target by J.J. Bittenbinder that has life saving funny and sad stories gained from 23 yrs as a Chicago police detective--good info.
Also, one of our boys was being bullied for not having a 'girlfriend' by some boys on his football team. So I quickly got to know their mothers who are nice which helped a bit. The boys are 'evangelistic opportunites' is one perspective I shared with my son. But I allow him to stand up for himself. I had to tell his team mom when one bully kid dumped out our gatorade because our son would not let him drink out of his bottle which is expensive for us (befuddles me why the kid would risk sharing someone else's germs). Sorta of like the Chinese proverb of hold your friends close but your enemies closer.
Additionally, no one picked on my husband after some big bully 'encountered' him in swimming class in middle school. That was the end of that.
The more I observe over the years (& experienced), the more convinced I am that mass education (esp. government runned) puts kids in an unnatural setting that does not produce a bunch of good fruit. In many cases, it seems to produce mindless, entitlement attitude, bullying citizens. :)