So far I don't think it has been confirmed that there even are any Japanese subs in SH4. However, I recall the sinking of Japanese subs in SH1 was a rather dull affair. They just sailed placidly along until you put a torpedo in 'em. They didn't try and submerge even if you missed or had a premature detonation. If there are Japanese subs I hope they are a little more awake.
