I guess OP is meaning LTD 50% marketshare, not weekly, that PS4 is currently quite often exceeding.
So, for the most difficult goal to be achieved, PS4 should outsell both competitors combined by more than 1.8M in 15 weeks, that is more than 120k/week lead on competitors combined on average for 15 weeks.
I really don't know whether it can do it or not, but I guess putting down the numbers can help people getting an idea of things.