GTA IV is the first game in the series with choice in the story, in the HD universe. This is also the first game in the series developed on its own engine - RAGE. The events of this part take place in the city of Liberty City (a city that was in other games in the series, including GTA III) - the worst city in America.
The game's plot is great (in my opinion, the best in the series), the atmosphere is simply gorgeous, and the gloominess suits the game. The graphics of the game are quite good (but there are problems), they look good even now, a decent number of characters come from the USSR, all the characters are designed perfectly. And it was this part that became closest to the CIS players. The game also has multiplayer, which is not bad, but the additions are simply gorgeous.
But the game also has a significant minus - optimization. Lags occur even on powerful computers.
In my opinion, GTA IV is the best part of the series (along with GTA: San Andreas)