The B4 is hardly well-armored so 2 or 3 hits and you are good to go. Often they place it next to the base sector so you also get to land a few hits on nearby units as well.
I've found though that placing it next to base sector makes it literally useless though. While it displays a pretty long barrage range, honestly it's way too unreliable at longer ranges. The only time I've ever gotten at least some mileage out of it was placing it in pretty risky positions, basically as far away from a mid VP as you would park, say, a 120mm mortar or such.
And the poor (understatement, it will hit miles away from the barrage circle) accuracy would be okay if it had decent area of effect. However it's pretty ineffective unless it hits something spot-on. I've seen it impact right next to an MG team and they suffered a little tick of health damage but that was it...