As we all know, the tentatively titled Mass Effect 4 will be a fresh restart in the series. There will be a new protagonist in a new adventure in place. However, Bioware mentioned nothing about it being a standalone title, or a new trilogy in the making.
For it to be the former is unlikely. Looking at the first Mass Effect trilogy, it simply says that the Reaper Wars can’t be wrapped up in a single title. There are just too much rising to action and plot twist in the arc.
With that in mind, we can expect Mass Effect 4 to be a new arc coming. Bioware did hint on the age of exploration and discovery to return in this next itinerary. As such, Mass Effect 4 will be more of an introduction that needs sequels to complete the rising action and climax.
On another note, half the fun in the first Mass Effect trilogy was with the save file transfer system. A trilogy is necessary to compliment this feature and Bioware is surely aware of it.
Nothing might be confirmed at the moment but it is safe to assume a trilogy coming with Mass Effect 4.