For unknown reasons, Rockstar Games has withdrawn from Steam the fourth part of the series and the collection of GTA IV Complete Edition. The game page is still available, but the price tag and the “buy” button is gone. At the same time, GTA Episodes from Liberty City is still available to buy.
Recall that earlier problems with GTA IV appeared in 2018, when due to the expired license for songs the developers had to almost completely replace the tracklist of Vladivostok FM radio. What exactly happened today is unknown, there are no official comments from the developers on this issue.
There is no fourth part in the Rockstar’s own lounge. In the catalog you will not find either the fourth part or the episodes.