To access the switch/open the exit on map level 4, COMBINE:

The exit switch is in a small booth on the west side of the
Automap.  Entry to the booth is blocked off by 2 doors: an
outer door which can be opened with a yellow key, and an
inner door which is opened by a switch.  The switch is
inside a small structure in the southwest corner of the
Automap, in the same general area where the exit booth is.
The switch can be seen through a window in the structure,
and is activated by firing at the switch, once, through the
window.  Use a pistol, shotgun or chaingun to activate the
switch, and the inner door of the exit booth will open.

Hope this helps. :)                                      

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