Selected Answer
Open the code sheet behind the worksheet on which you want the action. In the right pane you see two drop-downs at the top. Open the left one and select Worksheet. An event procedure will appear in the blank pane: Private Sub Worksheet_SelectionChange. Delete it.
Now select Change from the drop-down on the right. Another (very similar) event procedure will be instantly created. This time it's Private Sub Worksheet_Change. This is the one you want. Into this procedure paste the code below.
If Target.Address = Range("A4").Address Then
MsgBox "A4 was changed"
End If
Now every time a change is made in A4 the message box will be shown. Change the cell address to the one that should trigger your macro and replace the instruction to show a MsgBox in the code with a call for your existing macro.