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Typically Slot 4 on Battleships is going to be used for reducing Fire & Flood duration by 15%. Battleships can expect to be lit on Fire whenever possible by damage-hungry enemies.

Propulsion mod is more for Cruisers/Destroyers that have enough kick in their engines to juke some shellfire. While you can use that on a Battleship to generate some misses, the California is not very speedy/nimble.