Hi Weenie
For clarification.
Your code will work if LastRow cell value will be 5, e.g Row 26.
Try to avoid Selecting cells. This will "overfill" the range to complete the sequence
Sub Test()
    Dim c As Range
    Dim LastRow As Long, i As Long
    LastRow = Cells(Rows.Count, 2).End(xlUp).Row
    Set c = Range("D2")
    c = 1
    c.AutoFill Destination:=c.Resize(5), Type:=xlFillSeries
    For i = 5 To LastRow Step 5
    c.Resize(5).Copy c.Offset(i)
    Next
End Sub