I find it similiar and perhaps that is mostly due to the sound recycling. New diesel and electro sounds would REALLY have cut the mustard. You listen to that all day long.
Apart from that I find the change of scenery and scope of the conflict very entertaining. The whole theatre is just more interesting. To me, it became rather dull to sink cargo tin cans in the atlantic. IT was a brave struggle, but in the end you knew what happened
The variety in SH4 is bigger and I enjoy the mix of missions and "tonnage war". Im also VERY impressed by the ship models this time. And I love the waves. They are beautiful.
In the end, you move submarines around to sink world war 2 ships. There _IS_ a limit to the variation you can do. Perhaps too much SH3 make you feel that SH4 is already too much