When Bob Lazar went public with his story about UFOs at Area 51, he identified the site where they are housed as "S-4", located at Papoose Lake, about 10 miles south of Area 51. He described the bus transfers from the main Area 51 base to S-4 and the facility itself in great detail. He claimed there were seven hangars, with concealed doors built into the side of a mountain, in which the UFOs were stored.
The Papoose Lake area is part of the Nevada Test and Training Range. It can not be seen from any of the nearby view spots. But even on new high-resolution satellite images of the area there is no evidence of a facility there, or of substantial traffic between Papoose Lake and Area 51. There is a very faint dirt road, but it does not show any signs of frequent bus traffic. There is also no sign of activity (disturbed ground etc.) around Papoose Lake.
There is a site known as "Site 4", often abbreviated as "S-4", but it is part of the electronic combat range near the Tonopah Test Range (TTR). It is over 50 miles north-west of Papoose Lake. Since Lazar seems to have had some inside information when he came up with his story it is likely that he heard about "S-4" from his contact and used the designation for his story.