The unit reads:
The Blood Slaughterer will charge anything that gets in its
way, attacking ferociously until it is destroyed. They must be placed on Charge Orders, but may also fire in the
Advance Fire segment if not in Close Combat. Blood Slaughterers have a 180-degree forward arc of fire and will
target the nearest model it can see, friend or foe.

As its worded, the most likely thing to happen is that the 4 Blood Slaughterers shoot each other turn 1.
Shouldn't it be worded that it shoots at nearest friend or foe that is not part of its own detatchment?

Yeah, point. I think most would assume that