Get it from anywhere but Steam, with Steam you have go through extra steps to get mods working.
I don't hugely know anything about multiplayer but I'm sure some people on here can provide a better explaination about multiplayer. As far as I know, MP is down but still can be played with Himachi or Tunngle.
For mods, there is a download button up above next to User CP, then click on Silent Hunter 4. If you're not really sure where to start, I recommend taking a look at this
post, he's also got an Advanced mod list if you'd want a bigger challege once after becoming accustom to SH4, which what I did.