In "The Gathering", B5 used a series of thrusters to stabalize it when the assassin's self-destruct explosive knocked them out of position. Who says that B4 has to have actual engines? Just say that it has a whack of thrusters placed all over the non-moving sections of the hull, which are typically used to keep it oriented properly, but can be used to relocate the station if necessary. It wouldn't be able to move or accelerate very quickly, but it'd be enough to move the station out in an emergency. Most specs I've read indicate that it may even be able to survive a hyperspace jump, with a sufficiently wide-mounted jumpgate. Just make sure that the station has multi-directional maneuvering capabilities, and a very low top speed and acceleration. Just make sure that in mission, the model has no AI class assigned to it, and perhaps "no dynamic goals" so it won't go wandering off.