41 years ago today, the Salyut 4 space station was launched into orbit from Baikonur Cosmodrome aboard a Proton rocket. Salyut 4 was the first successful station of a new generation of DOS space stations (having followed an unsuccessful launch in the spring), with more power and more instrumentation than the first two DOS stations had possessed. It was still dwarfed by the American Skylab, but Skylab was now vacant in any case, the program effectively over after the last crew had left in February. Salyut 4 featured a third solar array, allowing it to power ultraviolet and x-ray telescopes as well as a range of space medicine research projects. Salyut 4 was visited by two crews, Soyuz 17 and Soyuz 18, and then visited by the unmanned Soyuz 20 in 1976 for a long-duration docking test that lasted three months. As one point of historical interest, there were three crews who attempted to fly to Salyut 4. The second, which would have been Soyuz 18, became Soyuz 18a because of a launch abort. Soyuz 18a had what was probably the most hair-raising survivable flight of any Soyuz crew, as the flight was aborted at T+ 288.6 seconds due to a failure of the core and upper stage to separate properly. The abort subjected the crew to up to 21.3 Gs, but they both survived; Soyuz is a remarkably rugged spacecraft, able to save the lives of its crew even when a great many things are going very badly wrong.
Salyut 4 remained in orbit until February 3, 1977, and was succeeded by many more Soviet space stations. However, as it had only one docking port, it was not capable of resupply, and so was also not capable of continuous habitation. That would have to wait for a later generation of space station.