I recently picked up a copy of SH4 and just completed my first war patrol.
I think the game is a huge improvement over SH3. In SH3 I could never manage to sink anything but lone ships for some reason, yet in my first SH4 patrol I sunk over 18,000 tons! (then I remembered I forgot to changet the realism settings, they were 5/100.

completing the last half of the patrol submerged was fun!)
I really like the cutscenes between patrols, they keep you informed of whats going on during the war. Getting radio reports of events like the japs invading Borneo is also really nice. I don't remember anything like that in SH3.
The game also runs better on my system than SH3 which is a surprise. It looks way better too.
I'm running it stock 1.3 right now but I might try Trigger Maru at some point later. Right now it feels way more like the original Silent Hunter 1 (which I loved, superb game) than SH3.
Mmm now time to kill more japs.