Apparently getting some of them to your settlement is getting problematic, where you send them, they take up a population space, but never show up. There's a 13+ minute YouTube vid on it that goes more into detail. Since I can't post the link, go to the search box and type: "fallout 4: how to get all 8 level 4 merchants (in depth tutorial)" and there should be a vid by a user named "JumpinProductions" who made it.