Found this on a site...
Mass Effect 4 could end up being a prequel. Casey Hudson, one of the producers for the game, posted to Twitter asking fans for their input on the timeframe, asking:
Parsing through your thoughts on the next #ME game. Would you be more interested in a game that takes place before the trilogy, or after?
At this point the fan response supports a sequel over a prequel. As reported earlier, Hudson previously took to Twitter to ask fans what they would want to see in the game, which is being developed by BioWare Montreal
So what do you guys think? I like the idea of a prequel set during the Rachni Wars, or maybe even a previous Reaper Invasion. That being said I would much prefer a sequel.