It isn't a prequel because it won't lead up to Shepard's story. Although it is set in the same Universe it is a different time.
I can tell you from what I remember before my contract with Bioware was done, was that they have planned the universe to be 4 times larger than ME3.
They were confident also in having a base set up on specific systems to help with fighting against "the bad guys"
But like I said last year and a bit, it won't be called ME4, that is just a temporary title being used because most fans called it ME4.
You will be able to see the events of the First Contact War and the war in the Skylian Verge against the Batarians. Allegedly :P
[Edited by DABhand, 6/10/2015 11:44:39 PM]
That is what I was hoping for at the very least, that the storyline will take place after the events of ME3. Watching the trailer it looks like we might get to see a civilization not based upon the mass effect field.