Well, you say you want to do a quad? That has 3 already. I wonder which gameport pin is for twist (rudder)? Because you could use that channel for your 4th throttle.
I don't know if that report is entirely accurate, either, because my analog gameport MS Prec Pro Sidewinder has 4 axes and 9 buttons, and it's not even using all the pins in the gameport!!
So there must be ways of putting more info into the existing pins.
As a second option: My motherboard came with a gameport panel for the back of my PC case. In addition to this my SB Audigy also has one. Thus I have 2 working gameports. If you just want to build one exactly like he listed, then build a second one, you could see if your motherboard has a gameport plug and add a panel in the back of your PC (or get a PCI gameport card, fairly cheap!)
Let us know how it works!!
EDIT: One thing I should add. I'm not too impressed with the quality of the work. I'm impressed as hell that he made it himself and it WORKS! However those wires he used for the throttle arms look very weak. If you could tweak the design at all when you work on it, and make a stronger arm, I would highly recommend it.