The idea that Spider-Man exists in Liberty City in GTA IV was widely debated in the early years following GTA IV's release. Many internet forum posts and YouTube videos portray the Spider-Man myth as a rare find, while other don't take it seriously and have shrugged off as a joke and have no belief in its existence.
Spider-Man was a very popular character back in the late 2000s decade, thanks to a trilogy of successful movies based around the character. As the release of GTA IV took place near the releases, it was expected that Rockstar would of satirized the film as they had with other movies in GTA IV. Thanks to Liberty City being based on New York City, the setting of the comics, many fans insist that the super hero was included in GTA IV as an Easter egg but there was never sufficient evidence to support it's existence. This led many to dismiss any idea of a serious bonafide Spider-Man and the rumors began to fade.
Modifications of Spider-Man were eventually created and became some of the first mods available for GTA IV and thus the fan base was satisfied. Only a few photos that seem legitimate exist but are all fake.
On the streets of Liberty City in Algonquin, a pedestrian on the phone can be overheard saying "Hello, It's your friendly neighborhood super-star." This phrase is identical to Spider-Man's greeting (minus super star.) This is an Easter egg but some people take this as evidence.