may say "END" periodically,
in that case you would need to check its value in the worksheet selection change event, then run the code when criteria matches like
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Range("ab4").Value = "End" Then ' copy cells
End Sub
if you need the cells to copy, even if there is no user interaction, then you may need to use application.ontime
in either case you may need to know if the cells were already copied during this period of ab4 value being "end", so it will only copy once during the cycle