The solution is to upgrade to release 5. OK this is a bit drastic, but if you get this message a lot, it is your best option.
Alternatively, close down any Notes windows you do not need at the moment.
Click on Window, and click on the windows 1-7 at the bottom and hit escape or the close button.
Another way to flick through windows is the use the Ctrl+Tab to move through the windows.
You can close windows by Ctrl+W, pressing Esc, clicking on the top right x of the window not the client, or by double clicking the right mouse button ( if you have this set up).