Boys are boys.
That's a load of gender stereotypical shite, at least until puberty. And then the testosterone simply changes the kind of fighting.
PEOPLE fight because they get angry with other people and think that you can win if you can dominate the other person, either physically or mentally. Personally, I don't think this is acceptable, and that everyone needs to learn to talk it out or just walk away.
Of course 18 months isn't old enough to talk things through, but it's old enough to learn the phrase NO HITTING and old enough to start learning that anger must be confronted not acted on.
Four years old however is easily old enough to know not to hit back. Explain that the younger is frustrated and angry and can't talk, so he has to be the big kid and not retalliate. A parent could be called on, not to tattle-tale but to help teach younger not to hit.
Getting along is one thing, but fighting when you don't get along is another issue.
is this a boy thing?
No, it's a person thing. I've got one kid who hits and one who doesn't. The younger one is the hitter and has a far hotter temprement.