Need help with AIU

I don't have a AIU, but I have one suggestion.  Don't use track power for uncoupling tracks!  I tried that on my Legacy system and it kept resetting my TMCC Direct Lockon. 

I don't have a Lionel uncoupler section to check, but I suspect you have 2 commons in the 4 wire setup. Look at the bottom/inside and see if 2 wires end up connected to the outside rails.

Or use an ohm meter to check.

It's also possible they use the extra wire to switch a set of extra rails for those old slide shoes.

The extra connection is for the accessory shoes.

Here's Wiring Diagrams for Lionel RCS/UCS Tracks.  Although this is for 027, the standard O tracks are very similar.

It the simplest form, you can just connect two wires to the uncoupling track if you just want the magnet to work and are not concerned about the accessory shoe connections.  If you want both unload and uncouple functionality, it requires two channels to mimic the two buttons on the original controller.