Grand Theft Auto IV is no longer available for purchase on Steam. As noted by PC Gamer, the game page is still up, but there's no option to add the game to your basket, and so far Rockstar hasn't provided any explanation why.
It's likely that this is only temporary, as the GTA series has faced similar issues in the past. In 2012, Vice City was removed from marketplaces over a copyright claim relating to its use of Michael Jackson's Wanna Be Startin' Something, but that seems unlikely to be the cause this time around - GTA IV was patched last year to replace songs whose license was about to expire.
Other potential reasons for the disappearance could relate to the game's ongoing reliance on Games for Windows Live, which requires keys generated by Microsoft. We've reached out to Rockstar for an explanation, but received no reply prior to publication.