I'll second Torplexed's comments. If you are familiar with SHIII and GWX then SHIV will definietly be different but in a good way. I think it fair to say that SHIII due to the very nature of the Uboat campaign puts the pressure on the defensive side as time passes(i.e. the evolving sonar/depth charge/hedgehog technology etc). SHIV, again due to the Pacific campaign, is more offensive in that (as Torplexed mentioned) the Japanese didn't utilize the convoy system as much and the anti-submarine practices were not to the standard of the Allies in the Atlantic. The US eventually held the upper hand in the Pacific but life was still dangerous on a Fleet boat.
Depending on how authentic you want the game to be you'll find it has it's own challenges. Just keep in mind that SHIV is not SHIII but they are (thanks so much to the modding community) great representations of their respective theatres of war.
