autofill in macro range is constant- how can I code to be a variable range?

I am trying to write a macro which will autofill specific columns. The macro will set the range from the start of my autofill to the end of my autofill as a constant range.
The problem I need to get around is the end of my range can always change each time I run the macro. For instance, the first time I run the macro I may only need to autofill from row 4 to row 15. The next time, I may only need to autofill from row 4 to 23 (because of user updates). How can I make the end of my range not be a constant address but variable?


Range("AR4:AW4").Select
Selection.AutoFill Destination:=Range("AR4:AW15") <---- 15 changes from day to day