Here's a follow-up question. The rules specifically state that you can't take an action that would reduce your workers below four at the end of YOUR turn.
However, there is at least one deal which allows the promotion of two workers.
A literal reading of the rules would allow a player to use this deal and drop below four workers as long as it's not on their turn.
I'm guessing this is just an oversight in the rules, and that you can't ever go below four workers at the end of anyone's turn?
Yes, that's correct.
You can't go below four workers at the end of any turn. Thanks for spotting it.