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
