excel - set a range of values to zero using if -

i want this:

a           |   b

===================

'blank'     |   1  12          |   4     'blank'     |   2  13          |  'blank'

i want check b column , if there number there ( number ) ,then fill corresponding cell (but blank(empty) cells ) 0 value.

i trying this:

=if( isnumber(b1:b4)  ,  ( if( not(isnumber(a1:a4) ) ) ); (a1:a4  = 0);""    )

but doesn't work.( have set else statement above "" )

if happy write formula in new column use formula in column c every cell c1=if(and(isnumber(b1),a1=""),0,a1) drag formula c2, c3, c4 etc.,.

Live one