I have a question: what exactly would a Y-cable do for a joystick on the control board? Could you use both sticks to control separate functions? What I am trying to do is use 4 sticks, plus a separate custom control box, but I can’t figure out how to get the fourth stick on. Does anyone have any ideas?
using a y-cable like the one in the white papers will allow you to plug 2 joysticks into one port. I believe it allows you to use all 6 axis and all 4 buttons. Be careful which port you use. The y-cable shown at Innovation First is designed to be used with port one. Notice that it can control functions normally controlled by other ports.
Each 15 pin input on the OI has 4 analog inputs and 4 digital inputs. Depending on the port, you also have 4 additional digital inputs (shared with the digital inputs from another port) or 4 led outputs.
There are many ways to do what you want. You could make a y cable like the innovation first one, and run two joystics off of one port. With 4 joysticks, you would have 2 ports availible for your custom control box. By doing this, you can’t use all three analog inputs of a joystick (only 2). You also only have limited access to either the LEDs or the extra digital inputs on the OI.
If you want to have access to everything imaginable, run all 4 joysticks into your custom box, and then run 4 custom cables out. Then you could have access to all 16 analog inputs, 16 digital inputs, and 8 LED outputs.
See pages 7-10 of the operator interface reference guides for the pinout. The possibilities are only limited by your imagination and how much soldering you want to do 
Thanks for your help! I think my devious plan is possible now…