Improve the performance of a loop

[querypk at gmail.com]

What is the fastest way to code this particular block of code below..
I used numeric for this currently and I thought it should be really
fast..
But, for large sets of data (bx and vbox) it takes a long time and I
would like to improve.

vbox = array(m) (size: 1000x1000 approx)
for b in bx:
    vbox[ b[1], b[0]:b[2] ] = 0
    vbox[ b[3], b[0]:b[2] ] = 0
    vbox[ b[1]:b[3], b[0] ] = 0
    vbox[ b[1]:b[3], b[2] ] = 0

and vbox is a 2D array
where bx is of form [( int,int,int,int),(........) ]