Now before we jump all over this guy, maybe we should get a clarification of what he's really trying to accomplish.
Using the shifter is pretty obvious, but it depends a little on what results you're after. Shifting to 3 gives you 1-2-3 and torque converter lockup on 2 and 3. As far as I know, it does not change the shift points, but it does change the TC lockup pattern. In "D" it locks up in 3 and 4, rather than 2 and 3. If you want to change the TC lockup characteristics, this would require some kind of switch, though the obvious thing to switch would be the lockup solenoid rather than the shifting.
So, if UltimateG has not been embarrassed into total silence, how about it? Is there some effect you're after other than what you get with a downshift?
By the way, to disable fourth with a simple switch would be very difficult. The AW4 is controlled by a pair of solenoids, which determine the gear depending on their combination. The solenoid combination for fourth is "off-off." This means that whatever switch arrangement you use would require you to switch solenoid #2 back on when the TCU wants if off for fourth. But you could not simply hard-wire it on without losing first gear. So you'd need to switch on the fly and remember to unswitch every time you come to a stop. About the only mechanical solution would be wire a relay that turns solenoid #2 on when #1 goes off, but returns control to the TCU when #1 goes back on. And of course whatever relay you use would have to have sufficient impedance and isolation that it doesn't load the solenoid circuits or feed back. In other words, fuhgeddaboudit!