Strange! Port #4 goes to the center vent pod. However, you can still get air out the center vents with port #4 bad, if the diverter flap opens (#6). What you can also do is turn the fan on maximum and apply vacuum to one port at a time, and observe the difference in output at the various vents. I'm starting to suspect a flaky pushbutton unit as the cause of your floor vent problem though, but if #4 doesn't hold vacuum, your center vent pod is dead (or the line is disconnected, or leaking). All the diagnostic information is in the climate control service manual on the CD-ROM. If you need the CD, drop me an email.

Oh yeah - to remove the climate control unit, remove the two Phillips screws at the bottom of the wood trim, directly above the radio. Then carefully lift the wood off, pulling the bottom outward, and the top will unhook - no force needed. Then there are 4 or 6 screws holding the p/b unit in - remove those, it pulls straight out. You can see the footwell flaps from there and observe operation while applying vacuum.