Formula Problems With Cells In Same Coloum

Hi

I'm having problems with getting a formulas to work . I Have attached my worksheet so you can see what problems is im having.


The problem is the formula in cell L20 is correct =IF(j20="","",J20/L4).
When I go down to next next cell L21 the formula changes to
=IF(j21="","",J21/L5). I need the formula as it drops down by 1 cell at a time to always be divided by L4 and continue each time a new line is added

This is =IF(J21="","",J21/4)
=IF(J22="","",J22/4)
=IF(J23="","",J23/4) and so on

As the quotation grows in rows the formula needs to grow with it

Hope this makes sense