It would kind of be like if four children who were fans of Harry Potter killed themselves.
What I mean by that is the fact that things like Call of Duty and Harry Potter are wildly popular. It's more likely than not that any teenage male in these societies has played Call of Duty multiple times. There are probably multiple million teenage boys who sign in online to play more than 2-3 times a week, especially on breaks. Vast majority do not harm themselves.
The game may not be helping their mental state, but it's not something to blame. I don't think teen boy CoD players are a majority of suicides and I don't think suicides are committed by the majority of addicted CoD players.
Many teens don't like calling for help. If they're simply docile usually they can probably just put on a facade to cover real depression from inquirers like parents or acquaintances. "We didn't see it coming." can be said about a lot of suicides, especially those of youth, I think.
