EDIT: Now that the game has been patched several times, it's much more stable and closer to what it should be. I
The interface in SHIII was very well designed and intuitive. One could select a station within the sub either by pressing a function key or clicking on the appropriate on-screen icon. Once this task was accomplished, a new sub-menu of commands would become available to the player and these could also be accessed by pressing the appropriate number key or clicking on the on-screen icon. For some reason (one assumes some idiot bean counter at Ubi read a middle management report touting the value of "streamlining the interface for greater ease of play" and demanded a dumbing down of the game to appeal to the mouth breathers out there), the new interface in SHIV has less options for stations, but the interface is more cumbersome. You can still select a specific station by pressing a function key or by mouse clicking (you can no longer get to most stations by walking around the virtual sub as you could in SHIII) but the sub menus are no longer mapped to keys. In fact, should you select a station via function key rather than by mouse click, the sub menu does not even update so you cannot choose any of the station specific commands as the on screen menu does not refresh. Even with the patches, this is still the case. I highly recommend downloading and installing the Trigger Maru 3rd party mod. ONe of the many improvments it offers includes returning to the SHIII interface. It makes the game much more user friendly despite the addition of new controls and stations.
Graphically, the game looks much, much nicer than SHIII now that the patches allow for FSAA and AF. The resolution is still locked at 1024 x 768 but with AA enabled, the game looks very nice indeed.. These graphics come at a price and if you enable volumetric weather effects, expect a big hit in frame rates..
Almost all sounds have been copied directly from SHIII which is fine as they were qutie well done. I recommend downloading some of the radio mods. Hearing actual WWII radio broadcasts really adds to the immersion factor and there are some excellent mods out there with hours and hours of news broadcasts, entertainment programs and music.
The game still offers up a dynamic campaign, much like in SHIII but throws a lot of time specific scripted events at you in an attempt to make it feel more historic. That's a nice idea, but it feels more like an over the top Hollywood action movie. Zeros and Bettys appear out of nowhere with far too much frequency and spot you automatically, no matter what the time of day or the weather. Getting strafed 40+ per patrol is not fun, it's annoying. The Trigger Maru mod does what it can to fix the number of planes, but there's only so much it can do.
The AI was totally broken in the initial release, but it's much better for things like convoys. Harbour AI is still wrecked. It's far too easy to sneak into a harbour and pick off docked ships. .
The game, once a bug ridden mess with missing features and poor optimization is now a much more polished game and I now rescind my initial score of 3.9. I say pick it up, install the Trigger Maru mod and some radio stations and join the Silent Service.