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property theSavedValues : {"Application Windows", "Mission Control", "Desktop", "Notification Center"} -- for example

tell application "System Preferences"
	activate
	set current pane to pane id "com.apple.preference.expose"
	tell application "System Events"
		tell window "Mission Control" of process "System Preferences"
			click button "Hot Corners…"
			tell sheet 1
				tell group 1
					set theCurrentValues to value of pop up buttons
					if theCurrentValues is {"-", "-", "-", "-"} then
						repeat with i from 1 to 4
							set thisValue to item i of theSavedValues
							tell pop up button i
								click
								click menu item thisValue of menu 1
							end tell
						end repeat
					else
						copy theCurrentValues to theSavedValues
						repeat with i from 1 to 4
							tell pop up button i
								click
								click last menu item of menu 1
							end tell
						end repeat
					end if
				end tell
				click button "OK"
			end tell
		end tell
	end tell
	quit
end tell