There is something called 'locked reference' in excel which you can use for this, and you use $ symbols to lock a range. For your example, you would use:

=IF(B4<>"",B4/B$1,"")

This locks the 1 in B1 so that when you copy it to rows below, 1 will remain the same.

If you use $B$1, the range will not change when you copy it down a row or across a column.