Based on this image and the commentary about it, I am speculating that ME4 will take place some time after the Reaper War. I also believe it will take place in the Milky Way galaxy.

When discussing this image the developers mentioned a few things. First, it was meant to invoke memories of things we have seen before. They specifically mentioned the LED sky. Which is to say, The Citadel. They also seemed to hint at this being a hub. With those two things in mind, it seems clear that the Citadel will not be in the next game. It may still exist for all I know, but it won't be in the game. This leads to my speculation that this could possibly be the new seat of a galactic government. At any rate I think this image points to ME4 taking place sometime after the Reaper War.
Also, when the developers talked about exploration, they talked about exploring "the" galaxy with no hesitations or pauses, as if they were simply taking it for granted that "the" galaxy was the Milky Way. The idea that ME4 would take place in some other galaxy has always struck me as preposterous, though it does speak to how badly ME3 ruined the ME universe that people would think this was necessary.
Based on these hints, I believe ME4 will take place either after the Reaper War, or in an alternate universe where the Reapers never existed, though I am inclined to not believe the latter. In either case, the setting seems most likely to be the Milky Way.
The simple thing that indicates this is a location from times set after ME3 is that it looks like a notorious big place someone would at least mention about from time to time if it existed in ME-1-3.
Most likely this is a place build after ME3, possibly a recreation of the original Citadel witch in canon outset of the new ME game could have been destroyed at the end of Reaper War in ME3.
Is there a better place to serve as a visual template for rebuilding, if all surviving races of ME3 wanted to build "new" Citadel? Well no - so no wonder this place looks very much like the original Citadel.
But I have to say that I do like what I saw today durning the N7 twitch stream.
Many would suggest we got barely anything.
I have to say it's good they gave us what they did, why?
The most obvious thing is the upcoming release of Unicorns and Dragons 3 (DA:I).
But IMO Bioware did something good - it learned from previous ME games, that teasing too much about the game just to please whining fans did in the end a lot more damage than good.
Mass Effect is a LOT about story and on top of that in the ME "4" Bioware also wants to put emphasis on that PLUS exploration.
The concept arts show us that in great deal - we good space exploration, riding mako on planets (even on a meteorite/dwarf planet!)
If they told us now a few key things about the plot/charactes - they would eventually spoil us the fun.
What they should have shown us - they also did. We know now that they want exloration/various location/exciting views.
There will be new alien races, just awesomesauce.
I only wish they shown us one more thing - how new-gen Turians and Krogans look up-close from all directions.
Frostbite 3 surely allows them to add lot more detail to these races, than previous engine did.