600 mp vs 1200 mp + population costs.
It is kind of like the cost of a printer and ink. The printer is really cheap and you think you got a great deal but then you find out that the ink cartridges are the same cost as the printer. This logic can be applied to the B4. The B4 is only 600 MP but the upkeep is 90 munitions + scout per shot. It adds up. The alternative is two ML-20s. It has an expensive upfront cost of 1200 MP but the upkeep is less. The ML-20 fires 4 shots within a decent radius. Coordinating both ML-20s means 8 shots within a radius which is guaranteed to kill something. There are other benefits to the ML-20 such as less reposition time.