User blog comment:N7EagleEye/New Mass Effect 4 Details/@comment-86.126.202.189-20130703211519/@comment-1440333-20130704003821

Bioware are not referring to it as ME4, and in fact discouraging fans from doing the same, because they say that suggests a direct sequel to Shepard's sequel. Basically they arent ruling out a sequel, but they could also do a prequel, interquel or parallel story.