Hmmmm...... alright, have a look at this:
34 months after the launch of the PS2 in Japan (March 2000-January 2003), the system had reached 50 million. The PS4 has been on the market for 26 months (on Jan 15). So, it would need to sell 14 million units by September. Can it do it? Anyone?
I could be wrong but I think Sony reported ps2 shipments back then as soon as they were created in the factory, unlike now when they have to be sent to stores.
So that's yet another advantage the ps2 had. Bottom line is the ps4 will keep up with it until 100m and then will probably fall behind.