Cooter, if you have any interest in the history of the U.S. sub war in the pacific, GET THIS GAME. The difficulty is scaleable, the strategy and action is pulse pounding, and the graphics, especially with the help of our talented modders, are beautiful. I think it can be had for relatively cheap nowadays as well.
Even if U.S. subs aren't quite your cup of tea, give it a shot. I was always a WW2 fan, and was looking for a new game to play when I ran across a SH3 review that said it was great. I gave it a shot and became hooked, and not only that, but I became a U-boat history fan as well. Be careful...the same may happen with you and U.S. fleet boats with SH4!!!
I went from SH3 to SH4 and it really prompted me to start reading a lot more about U.S. boats and now my bookshelf is stocked with almost an equal amount of U.S. sub books as U-boat books.