I don't think I need to speak on the masterpiece that GTAIV is, so I'll just focus on the Steam Deck part of it. This game, after installing FusionFix, runs like an absolute dream. It's a less than 5 minute setup, and I highly recommend checking out TJGM's guide on YouTube. There really is no better way to play. Performance with no special undervolt/power limits is perfect. Limit to 45hz, set all graphic settings to Medium (some tweaking optional) and you will not once see the framerate drop under normal circumstances. Power draw is nothing crazy, often times dipping to 12-14W when in cutscenes, which drastically helps this game reach about 3 or 4 hours of playtime before needing a charge. I couldn't ask for a better open world gaming experience on the Deck, and the mid-2000s pop culture time capsule that is Liberty City still holds up to this day.

Also, if you haven't yet, play Lost and The Damned. Very underrated expansion that often goes overlooked.