It has already.
It sold 3.4m last year (confirmed by IFPI) and accumulated 603k sales "points" on mediatraffic until it fell off the chart at the end of March. (The first post on this thread is missing points from week 13.)
Given that mediatraffic underestimated the album's sales last year (they had 2.9m instead of 3.4), it's pretty much a given that BTD passed 4 million sales at least a couple weeks ago.