August - City Break

None if you are not trying to get a break from hot weather and crowds but Istanbul would be my choice

Istanbul will be very hot in August and Croatia maybe too.

I would opt for Lisbon and if you have 6 days not 4, take day trips to Cascais and Sintra. Plenty of history and good food and wine.

My second option for 6 days would be a combination of Split and Dubrovnik, flying open jaw. Ljublijana doesn’t need 4 days and isn’t that easy to combine with Split or Dubrovnik, despite its relative proximity.

But Ljubljana would give you some access to the Julian Alps if you rented a car. Might be cooler there?

From that list i vote for Ljubljana (cooler)

Since they will all probably be quite warm in August, I wouldn't use weather/temps as a deciding factor. So, if you leave that out, any of them would be great for a 4-6 day city break with options for day trips. If I had 6 full days I might consider 3 in Dubrovnik and 3 in Split.

A break from what/where?

Without a bit more context, to have as a guide, making a recommendation is not much better than just picking one out of a hat.

From that list i vote for Ljubljana (cooler)

Why those places? In August?
Read Joe's post - we need more info.
In Europe in August, I would choose Stockholm, Copenhagen, or Oslo.

In Europe in August, I would choose Stockholm, Copenhagen, or Oslo.

It's possible that budget is more important to the OP than temperatures in August. We need more clarification from the OP in order to provide helpful opinions.

Actually I would spend the 4 days in a trout stream