If you'd like an easy discussion with your developers, here's my usual approach. Databases are really just backup containers. If the information in these four databases needs to be kept in sync on data restores, then they're one app to the database layer. If not, let 'em loose.
It's hard from a schema/structural standpoint to decide one way or the other as to how this should really be organized, we just don't know enough.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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