Auto-Fill

I currently have this code ( which works perfect ) that was written by someone else. It performs an auto-fill in column "A".

I need to learn how to adjust this code to do it for other columns, but do not know currently.

Maybe someone could highlight of BOLD the adjustments to direct the code to the column.

Sub fill_rows_A_4()
Dim arrTmp As Variant
Dim lngRow As Long
With Worksheets("sheet1") 'adapt
arrTmp = .Range(.Cells(1, 1), .Cells(.Rows.Count, 1).End(xlUp))
For lngRow = 1 To UBound(arrTmp)
If arrTmp(lngRow, 1) = "" Then arrTmp(lngRow, 1) = arrTmp(lngRow - 1, 1)
Next
.Range(.Cells(1, 1), .Cells(UBound(arrTmp), 1)) = arrTmp
End With
End Sub