Multiple City Unrest Penalty?

It's a flat rate = 3 x #ofcities

how to counteract, make sure you have several fortresses to reduce your unrest globally.

How does that work - looks like you need a city upgrade to reduce faction unrest?

There are several city improvements that reduce unrest, cleric, bell tower, etc

Keeping a hero or your sovereign stationed in the city also helps.   A Commander helps even more when given the Administrator trait.  But that sucks since they should all be working out in the field.
When a fortress achieves level 4 you can choose having a Prison to reduce unrest globally by 10%.

The "cost" for the -10% global unrest is -1 growth in that city.

But to be honest, since you're at level 4 already - you probably have a fairly decent enough city already.

(someone make a mod where I can use prisoners for soldiers)

there is also the lvl 5 fortress specialization called "Onyx Throne" that will decrease the faction wide unrest by 30. So one lvl 5 fortress can counteract the unrest penalty of 10 cities. I really like this mechanic because it makes city specialization and city growth much more important.

Unrest also effects research and production. I am not familiar with the formula, though.

Unrest gives a flat reduction to production and research. A city with 20% unrest provides 20% less production and research.

I think the -1 growth refers to the level 4 fortress upgrade.

and if you ever lose that city...

Which is why you defend and and give it the respect it deserves.

Yes, the Prison upgrade decreases unrest by 10% and growth by 1.