I agree, it is indeed playable....but its dull :-(
Its very nice to look at but it lacks the challenge and excitement of the Atlantic campaign, which may be historically correct and simply a reflection on the pacific war itself. From what i've read the Japanese weren't that effective at anti-submarine warfare and the US didn't rate submarines as highly as the Bundesmarine.
Also, I just don't feel "involved" in SH4, not like SH3 which is far more immersive, more nail biting, more challenging and less like a turkey shoot. I also feel i'm in a U-Boat in SH3 but dont at all feel like i'm in a US sub in SH4.
After spending time with both, I'm returning yet again to SH3 but will probably do a mission or two with SH4 every time i need some eye candy.
Roll on SH5 and back to the Atlantic please!!