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Unfortunately, I won't be able to create four panels in the city creature recruitment menu; perhaps someone else knows how, but I don't.
However, I can share an idea:
The possibility of recruiting additional units already exists.
You simply need to design the creature upgrades in a way that creates a loop between them. The first creature upgrades into the second, the second into the third, the third into the fourth, and the fourth back into the second.
This way, you buy Knights, upgrade them to Paladins, and then, through a series of upgrades, improve them in a cycle, turning them into Knights of Isabel, Green Knights, and then back into Paladins.