What GRJ said above about TIU channels and the PH-180, he added some more very good advice too.
My thoughts were that one PH-180 has a 10 amp total power output, and as GRJ says above, that could be plenty of power to operate 2 (or even more) TIU channels depending on your load. If you only have one train per loop, then the PH 180 should be plenty for at least 2 TIU channels / loops of track.
Then for conventional you could use the PH-180 on your two variable channels so you could have conventional control if you needed it and then run the 2 fixed channels with the handles of your Z-4000 if you needed conventional operation there. The variable channels can be set to fixed for command operation. The fixed channels are fixed only and can not be changed to variable. For command, those channels can be set to 18 volts with the Z-4000 handles and you're ready for command operation.
And to stress what GRJ also pointed out (very good point) combining power on TIU inputs is ok, but it is NOT ok to combine the TIU outputs, those must all be kept isolated from each other.
