Welcome the forum!
I have played DW/SH3/SH4. So I will pass on some comments.
SH4, as you know, is in the WW2 era. Nice graphics and a good campaign engine. Game consists mainly of hunting down convoys and sinking as much as you can. With some photo recon missions thrown in.
Different sub types are available and newer equipment becomes available during the game. Multi player is there, mostly wolf pack against convoy type of play. I tried Multi Player and did not like it, game is much too slow without the time compression you have in single player.
All in all not a bad game. I played it for a while then, for me, it became repetitive.
DW is a late cold war era. Graphics are functional, but not pretty. It has a campaign, but not a campaign engine. Lots of single missions have been made available by the DW community. The main focus of the game is a little hard to pin down as it depends on which platform you are using. If you are using the sub then there sub vs sub, sub vs surface with some recon missions available. As you may know you can also chose to operate an FFG, P3 or Helo.
The single player missions are OK, but once you successfully complete them they have limited replay value.
For me, where DW really shines is in the Multilayer side of things. It can be hard to find players, but once you do the game is a lot of fun. Multiplayer is what has kept me playing for a long time.
If you read through this forum, you should find lots of comparisons between the two game.
Have fun !
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Happy Diving !
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