Evidence 1:
The first piece of evidence lies in a radio interview with a couple people from Rockstar. They talked about the Lost and the Damned and The Ballad of Gay Tony DLC exclusivity, but when presented with the question about GTA V’s existence this is what they had to say:
Evidence 2:
Spreekt voor zich nu denk ik.
Evidence 3:
This is a billboard spotted in Grand Theft Auto IV. Depicted on it is an add for an airline promoting a trip to Los Santos for $299. Looking familiar? Well it should. GTA III had billboards containing promotions for Vice City, which ended up being the next location in the series.
Evidence 4:
This image was spotted on the Rockstar Social Club site. Printed on the ticket is Mr. Niko Bellic. So obviously it is for him and it makes sense since the billboard above was in GTA IV. Another interesting note to make is that the ticket is for a trip from Liberty City to San Andreas.