I think Jon has a good suggestion. If power fails during a 4 sec exposure, check if the shutter closes or stays open. Probably it stays open. That being the case, load up your film and get it to the place where you need it to be, put on a lens cap, set a 4 sec exposure, pop out the battery some time in between 0-4 sec, and then use your lens cap to control the exposure.
Experiment. Have fun.*
*I hereby deny responsibility for any adverse results that may affect the camera, the film, or the photographer....
P.S. Can you pick up an inexpensive, all-manual, Minolta-mount camera?