Well, they've held #4 for the last couple of days, so even if the numbers dropped, at least they haven't dropped much in the rankings.
Since Momusu has seemed to have been gaining more teenage fans lately, I can only hope the weekend will bring a bump in the sales from students that didn't get the chance to buy it earlier due to school.
While the older, more diehard fans usually buy their copies on the first day or pre-order them, maybe the newer, more casual fans wait for a convenient time to buy theirs.