Keep in mind that aside from people like Veronica and Christine, the West Coast Brotherhood isn't really made up of good people. The Brotherhood in DC had the people in mind, but the main Brotherhood and DC's Brotherhood Outcasts only wanted technology.
Now, supposedly, Fallout 4 takes place in the Commonwealth, which is meant to be the most technologically advanced place left in the world, and they're not particularly keen on sharing their tech (See: Fallout 3 - The Replicated Man). The DC Brotherhood may have made its way north, or a more traditional group of Brotherhood from the west may have traveled there. If it's the latter, the Brotherhood may not be the good guys this time around.