S4 (AKA site-4) is a hodge-podge of radar sites and who knows what else located south of the Tonopah Test Range. The area is visible somewhat from Brainwash Butte, and from locations further south such as Clif Point and Mojo Mound. I have only seen ordinary looking fighter aircraft fly over the site, but given the remoteness of the site, virtually anything could fly over the area undetected. Unlike Groom Lake, which has the ET Highway and the town of Rachel nearby, S4 is far away from roads and population.
Scanner audio from an exercise at this area can be heard here.
This is an interesting photograph. The first thing to notice is everything gets lighter by the telephone poles. Perhaps there is a wire fence at the border. The criss-cross of wires makes little sense. The large white dish is probably communications rather than radar.
The flat panel at the left is the Russian 96N6E, based on the shape of the panel.
The radar on the berm matches the Russian 64N6E.
Both radar are shown here.
Some other flat panel Russian radar links:
http://www.geocities.com/kish_yu/sa12.html
There appear to be small mounds of dirt in front of these buildings.
Note the buildings in the background. There are buildings even further back that these but they were lost in the haze.
This is the same area but wider, photographed in 2007. A higher resolution jpeg2000 version can be found here.
Probably fansong radar. E/F band on left and G band on right.
http://www.globalsecurity.org/military/world/russia/fan-song.htm
Looks like the fansong radar, except the dishes are as tall as a building.
This is located at the end of the runway on the dry lake. It may be just a visual/radar marker.
This appears to be a transmitter site located up on a hill overlooking the area.