I haven't tried it but I loved SH3, I don't know if I would enjoy 4 as much as it's set in the pacific on a US sub.I liked being a U-Boat commander for some reason.
[youtube]http://www.youtube.com/watch?v=1wfRCMv3sNo[/youtube]
I read on a subsim forum that there was a guy doing a mission in real time,sleeping with the pc on next to him and waking when there was an alarm lol, takes all sorts.