You're entirely right that the age difference means they are not truly playing together; your daughter is like a doll to them -- one they can scare, or pick up and cart around, or tell things to in order to get a reaction.
My first thought was what someone else said: Build a fence, pronto. If you rent, that is an issue, but it's worth asking the landlord. If you own, bite the bullet and find the money (shop around too for a deal - some companies offer coupons in the yellow pages or online, on their web sites). If the family comments on the fence or on the fact "our kids can't come over to play any more!" just smile sweetly and say, "Oh, we've wanted a fence for a while (because you have --ever since these kids started invading your space!). And we want to protect little Sally from wandering off. It also might enable us to get a big dog for protection, to bark if someone comes in the yard...."
OK, that last bit was rather mean of me, I admit. But if they are acting very weird, you could indeed drop on them the idea that you might just get a big old dog to run around in your fenced back yard. And make certain that you have good gates built in with latches they can't work on their own to get in. Once you have a fence up it is far easier to say, "Sorry, but we can't allow you inside the fenced yard unless we set up a play date and ask you. It's for your safety."
Finally -- at their ages these kids are going to get bored with your daughter pretty soon, and move on to find other kids closer to their own ages. But a fence prevents any issues especially if you ever put in a swing set or other equipment down the line and don't want kids to wander in and use it at will.