I really like skymningen's answer, but I think there is another side.
First, at least here in the US (can't speak to other parts of the world) there is a strong "Anti-Bully" movement. So strong in fact that children end up missing out on valuable life lessons. Being bullied a little bit is just part of being a guy. It's how we develop some of our social skills, and to a very real extent how we learn some of our limitations. For example don't whack the big kid on the head, he will hit you back, then every one is hurt and sad.
Also, a lot more these days then ever in the past, kids are under the protection of their parents a lot more then they used to be. By 9 years old I was playing out side with friends "without" parental supervision. Now that doesn't mean that our parents weren't keeping an eye on us, on some level, but they were no where near as involved as parents seem to get today. As interactions move into the house or are limited to the smaller back yard, as parents we need to let some things go, and just accept that kids are kids.
The trick is trying to decide when an event or events amount to bullying. I believe the key word here is "No", or "Stop". When the child feels like they have no control over the situation, that's when the real problem starts.
So if two kids are standing there smacking each other with hot wheels tracks, laughing and having a good time, just make sure the guide pins are out, and get the iodine ready.
If one kid is smacking the other kid while he sits there going "stop it" or is crying, ok, that's Bullying.
Again it's about control of the situation. As long as everyone involved and is having a good time, and every one feels like there in control and can stop when they want to, then it's just kids trying to figure social stuff out.
Now, as parents, or even just adults, what we need to do is a two part check. First is there going to be real harm done by the activity? Just because everyone's on board doesn't mean the hot wheels track fight should be allowed to escalate to "lets use the knives in the kitchen fight". For the most part this is just common sense. The next check is rather or not the child in question feels like he is in control, or does he feel like he has no choice?
This is tricky. As adults we learn to put up with things in relationships because we want the relationship. We tolerate "that one thing your wife does" because we feel it's worth it overall. That's an important social skill. It's one that kids need to develop. At the same time though, as adults we need to teach that there are alternatives. That there is a difference between accepting a few "negatives" and the relationship being overall negative.
With that in mind, many parents have a tendency to pick their kids friends. In many cases directly, go be friends with this group of kids because these are the people we adults like to hang with. But, also passively, this is our church/school/day care/whatever. In these cases your removing the child's ability to be in control. So in these situations it's important to remember that if someone smacked us with a hot wheels track, we would just stop going to the movies with them but because the child doesn't have that choice they may be just trying to make the best of a bad situation.
So the solution. More kids, and listening to what the child wants to do. If you don't like that group of kids, well that probably shouldn't be your call (unless there is a real tangible danger). Instead try to steer the child to more "likable" groups. Join groups and social settings for kids. Give them a larger pool to choose from. I don't want to go to after school arts class anymore, I don't like it. Ok, how about after school pottery class, or after school music class.
Once the child figures out that they can have friends without the pranks, they will have a larger toolbox to deal with the problem group. Even if "deal with" is "I don't like playing with Tommy, he's mean, can't I go play with Billy".
Now you state that you can't have other playmates. This is it, this is the group. These are the only four kids on the island. They have to get along. While I don't think that is realistic, it really limits your options. I do think there are some kids you can't get away from. School mates, siblings, etc. But I would really re-examine why things are such a closed system.
Assuming this closed system, where there are no options for more playmates, then your best bet is to arm the child with the tools they need to "beat" the "bullies". This gets tricky again, because the "bullies" are not being physically abusive, so you can't teach your kid to just punch em. Instead you have to teach the child how to channel the anger and distress into something more positive. Careful probing, "Why are you angry?", "What has upset you?" can help get to the route of the cause.
"I'm angry because I never get to win at hide and seek. The others cheat!". "Ok how are they cheating?". "They change their cloths and then claim I didn't catch them." "Ok, lets try playing with a Polaroid, so everyone gets a picture of themselves, and when they get cought, they have to give the seeker the picture. That way they can't cheat and you can prove you cought them.
Again it's about teaching how to avoid the problem. If you can't avoid the people, then avoid the situation. We do this as adults. "I hate to go to lunch with Bill, he always gets way to friendly with his food.", but you have to, so you sit on the other end of the table.
If all else fails ban the problem activity. No more hide and seek. Ok, No more play dough. Ok, no more board games. Ok, no more TV. Everyone just sit in a circle and don't talk, your only allowed to play boring games cause that's all you guys can manage without getting in trouble.
So, as a summary:
- Boys will be boys, make sure your not suppressing that
- Make sure the child in question has control and can say no.
- Play with more kids, let him compare experiences.
- In forced groups, give him the tools to beat the bullies.
- Step in and be the adult, as a last resort.