It does indeed have MP. A new an improved MP over SH3. They call it the adversarial mode. Neal is going to be posting a part 2 to his SH4 preview, which will detail a little more information on the MP side of the game. It's coming "soon."
Here's an excerpt from a dev chat from 01-Jan-07. The devs had the following to say:
"
SH4 was designed with both single player and multiplayer in mind, as both parts are very important for us. True, Silent Hunter is more of a Single Player experience, but we believe that the new adversarial gameplay mode will change multiplayer in a significant way."