Another question. I read on the wiki that at the end of third era, dragonstar was split in half after the war of bendhr-mak. Half in skyrim, half in hammerfell. Since it isnt on the game skyrim, am i correct in saying that the redguards have retaken the city by the fourth era?

It means half was ruled by Skyrim and the other half by Hammerfell.

Anyway all the land that Skyrim had won was given back to Hammerfell after the 10 year interregnum was ended and Uriel Septim VII returned to the throne.