And the fact that NATM 2 had
1) shorter running time ( 105 min as opposed to T4's 130 min )
2) played in more theaters including IMAX
I haven't seen T4 yet so i can't comment on how the movie actually is however these factors do also play a (big) role in the BO performance of a movie.
Also i can't believe that Terminator 4 costs 200 million. Aside from Brian Singer's assraping of Superman Returns , "most" 200 (plus) million dollar movies do actually show that that they cost that much . SPider-man 2 & 3 , POTC 2 & 3 , Narnia 2 , Transformers 2 , King Kong.
From what i've seen in trailers etc , i just can't buy that Terminator 4 costs that much. I'd actully go for a number between 150-180 million