If Halo 4 released today you could expect roughly 495k WW based on a 120% increase (BTW I messed up last weeks numbers...it should have been closer to around 480k)
This week total Halo 4 pre-orders are 587% more than Halo Reach was (Down 35% from last week) and weekly Halo 4 numbers are 25% more than Halo Reach was.