Leave it to Rockstar to keep legacy games alive. Not only have old Grand Theft Auto titles been ported to mobile, re-released on all popular digital distribution services and made available on the current gen consoles (such as GTA 4 on Xbox One, via backwards compatibility) but these older game still get minor updates every now and then, even if we have no clue what said updates do.
Case and point, less than a week ago Grand Theft Auto 4 and its Episodes from Liberty City edition containing all DLC both got a strange 579.35 MB update. Now, in this day and age when most games skirt and some surpass 100 GB in size, you can't fit a whole lot into less than 600 MB, and no noticeable change has occurred in-game as far as users know.
This, of course, spawned a torrent of speculation.
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The first, and possibly most obvious assumption was that the game was given an update to look better on the Xbox One X, however this falls apart on multiple counts. First of all, each game that is Xbox One X enhanced is tracked on an official list, and GTA 4 isn't on it. Second, how would such an old game, which doesn't look up to par with today's standards anyway, be given such a minuscule update for better visuals? It simply doesn't add up.
A second logical assumption is that it's being primed for the date when a batch of licensed music contracts expire with Take-Two, meaning the use of those tracks in the game would no longer be legal, and this update is removing those and possibly replacing them with newer ones which are still licensed. This has happened to GTA games in the past, so it wouldn't be unprecedented.
Another possibility is that the update is just a minor performance optimization update. Even though backwards compatibility works as well as it does, there are still some performance issues with older games that don't play nice with the new hardware for which they weren't developed. Maybe the changes haven't been discovered because they're all under the hood tweaks and we'll never know exact details.
Should anything concrete about this mysterious update be unsurfaced, we'll be reporting on it - in the meantime, keep an eye on your copies of GTA 4 on the Xbox One if you have it, and try and find any changes.