i believe gta iv is considered a great game by many ppl. Those who bought it resold it creating more fans than the charts tell. It sold 20 million because theres 20 illion gta iv fans

Hype. Lots, and lots, of hype.

I mean, if you could materialize the hype, you could fill a couple hundred thousand cargo ships. And even then, post-release gameplay and word-to-word gave it even more hype.

Oh, and Rockstar are pretty bad ass.

I believe this game could hit 50 million! There I said it.

Why you ask? I think they will double-dip a lot of sales when they eventually release PC/PS4/Xone versions. About half of my friends who already have it say they would happily buy it again for a format that gives them 1080/60fps.

GTA5 never actually had a release date til Sept 2013, that spring 2013 was BS b/c it never revealed a date. GTA4 on the other hand sold so slow b/c it was highly anticipated, it was delayed from Oct 2007, to April 2008, & most fans was expecting another San Andreas, & it was too realistic.

Marketing
Install base Edit: also both PS3 and 360 are still selling well whereas when GTA IV was released the Wii was really important.
San Andreas / not so gray like GTA 4
Genre is more popular now (Skyrim another example of doing a lot better then oblivion)
More definitive online


That is about it right there.

It was hyped beyond your comprehension.

It's a AAAA title ?

Good game, good advertising, bigger install base, good scores, good fan reception.

While GTAIV had most of these things the install bases were lower and fan reception wasnt as good as the review scores say.  On the other hand GTAV has crazy good fan reception right now.

Because people are on dope. Kidding.

GTA5 does nothing the best, but it does everything well enough, except for flying which is probably the worst I've experienced in any open world game. Maybe because of this and that people don't see a new GTA every few years that sales are higher.

Also, what I think is a major reason of the high amount of sales is my theory on Rockstar holding back the online to attack used games. Sales could also be as high as they are because the first two weeks no one was selling their copy of the game, so people who wanted to buy it used had to either not buy the game or buy the new copy. Even during the time the online wasn't working, a lot of people still gave it the benefit of the doubt so add more time to that two weeks where there's less used copies available.

Cos it's a crime simulator.
People want to live out fantasies of violence and mayhem, GTA allows them to do so.