Cope-
You're definitely doing a lot of things right by having feeders every 6-8 feet and by separating the accessories onto a separate MTH Z4000. These two steps ensure that the tracks themselves are getting as much power as the ZW can put out to the rails, and that the power flow across the layout is even (i.e. no dead spots).
I'm assuming that each main line on your layout has its own dedicated 180 watt brick and corresponding channel on the ZW; if so this means each mainline is receiving a maximum of 180 watts. This is suited well for powering two or three locomotives with sound and smoke units, with some power leftover for illuminated cars. The fourth locomotive is definitely pushing the transformer beyond what it can safely supply to the track, hence the overload.
There are two ways to mitigate the overloads. One would be to simply purchase a larger transformer, and the other would be to subdivide your tracks further and add transformers to the subdivisions. Unfortunately there are no commercially available O gauge transformers larger than 180 watts (even the new ZWs or a Z4000 can only supply a maximum of 180-200 watts per throttle), and since you do not use TMCC/Legacy, the option to use a TPC unit (which allows to connect two 180 watt bricks in parallel to the same track) isn't applicable to your layout. Thus, subdividing your layout into power blocks/districts is the best way to go.
To sub-divide your tracks all you need to do is electrically isolate a section of track by removing the center rail pins at both ends of the section, or simply cutting two gaps along a stretch of track (if removing pins isn't feasible). Since your tracks are fed by common bus wires, be sure to disconnect the wire providing power to the center rail along your block, as you do not want adjacent power districts supplying power to this section.
Once the blocks are created, connect a separate transformer directly to each section of track; since the ZW can support 4 180w power bricks, it would probably be best to buy two more bricks, connect them to the ZW, then run wires from the ZW to the isolated sections. Be sure to connect the center rail feeder wires to the new transformer too, to ensure even power flow across the block.
With this setup your trains will run as long as there is power, but each block/district will have adequate power to drive one train (versus one brick trying to drive all four trains at the same time).
I know this sounds like a lot to digest, but keep asking questions as we're all here to help you!
-John
