It's been a long time since I've been here, but I am very happy to see this mod is still alive and a Work In Progress. I take off my hat for you, Rob.
After a very long hiatus, I have taken up playing some Battle for Middle-Earth again and I too have encountered the bug with the Black Gate map. Maybe I can give you a hint as to where the problem lies.
You see, it seems to appear whenever I use my units to stand behind the Black Gate, killing off all Orcs and Co. before they can sally out. So, my ideas on the matter are that the third wave sets of a trigger when sallying out to initiate spawning for the fourth wave. Can that be correct? However, I don't let them sally out and therefore they cannot trigger the fourth wave.
I hope that this helps you out a bit, Rob. Seems like the least I can do.