ME4 will be set during the final years of the Human/Turian conflict known as the First Contact War.
You play as Sam (Samuel or Samantha) "Buck" Rodgers, a human pilot who crash lands on a hostile, alien planet and is forced to cooperate with the very enemy you were trying to destroy when you were marooned -- the dirty Turian scum who shot you down and killed your best friend Dak (Dakland or Dakliona)! Revenge soon takes a back seat to survival as you and the Turian pilot learn to first communicate... then eventually love.
Long story short: The Turian gets pregnant and dies, you raise the Turian child as your own, you end up saving the kid from Human slaver/strip miners and eventually accompany the child to the Turian homeworld where you teach both Humans and Turians to look beyond their differences, end the war, and become allies -- and, more importantly, friends.
The multiplayer consist of a persistent hub tower containing DLC vendors and public 2-4 player Quasar pods (no match making - first come, first serve only).