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Although the topic is probably dead, I'll still add my own thoughts; maybe someone will read it and find it useful:
What would I use the fourth column for? To recruit neutral creatures related to the faction. There's no need to make them look like alternatives. They should be completely different creatures. I think the idea is worth considering.
Personally, I did this with the third column. I removed the obviously useless creatures, replacing them with completely new ones (not everywhere, where I liked the creatures, I didn't change them).
For example, in the Academy, the third column is completely replaced:
Gremlins Saboteurs - with Fire Elementals
Elemental Gargoyles - with Air Elementals
Obsidian Golems - with Earth Elementals
Battle Mages are moved to the second column, the third - Water Elementals
Viziers of Djinns - with Thunderbirds (the same Air Elementals, but with lightning around them and new abilities, and they throw lightning 2 times)
Rakshasas - with Manticore with a lion's face (although they could be in the second, and the regular ones in the third)
Thunderers - with Atlanteans (giants with a water element)
Also, the creatures of the Necropolis are completely replaced in order: Bone Mages (minor damage), Mummies, Marsh Creatures (a ghost of a woman, can shoot once), Ghouls (a mix of zombie and vampire), Necromongers (a Necromancer model on a hellish steed in corresponding Necropolis colors), Death Knights (well, everyone has seen them). And for Astral Dragons, I added a deadly gaze and incorporeality.
I also replaced everything for the Orcs. Everything is taken from NHF (trolls, wild griffins, wolves, troglodytes, etc.).
For the Elves, I only replaced Light Unicorns with Pegasi (they can heal like Paladins, and the combat ones can shoot once, like Jarls), Emerald Dragons with Golden Dragons, and placed Phoenixes in the third column.
And so on. Accordingly, all the new creatures spawn on any map, as they are a complete replacement for the original ones. And the removed creatures are placed in the neutrals.
Of course, the characteristics of the new ones are adjusted to the old ones. But some spells and abilities have been changed.
I told you about my idea and how I implemented it in my own way. Perhaps there is a better solution.