Water tower is INCREDIBLY buggy on this mission once parked, that's why I put it on a single spot for the whole mission, once the train has been driven away. Use the medic to immediately cure the train driver (he is always lying at the same location) then bring him asap to the ambulance. A little trick with portable cannon through the window at the beginning can save few seconds of extinguishing in the first building. Both portable cannons must be ready to extinguish when the water tower is near the boat.

Time may be slightly improved with better portable cannon positioning and overall micro-management. I am not sure if sub 4min is possible with this strategy.