Which world cities with more than a million people have exactly four letters? You are invited to name some selected smaller, more famous, ones as well.
I suggest to change Lvov to Lviv, as Lviv is the Ukrainian name, and it's a Ukrainian city. Lvov is the Russian name. Wikipedia also uses primarily the name Lviv.
Tula, Russia is over 500.000 inhabitants. Tver, Russia, over 410.000 and from my humble opinion, they should be there. Hamm, Germany has population over 180.000. As mentioned in one of previous comments, Graz and Linz are must have.