Salyut-4

Salyut 4 (DOS 4) (Russian: ?????-4; English translation: Salute 4) was a Salyut space station launched on December 26, 1974 into an orbit with an apogee of 355 km, a perigee of 343 km and an orbital inclination of 51.6 degrees. It was essentially a copy of the DOS 3 (or Kosmos 557), and unlike its ill-fated sibling it was a complete success. Three crews attempted to make stays aboard Salyut 4 (Soyuz 17 and Soyuz 18 docked; Soyuz 18a suffered a launch abort). The second stay was for 63 days duration, and an unmanned capsule, called Soyuz 20, remained docked to the station for three months, proving the system's long-term durability despite some deterioration of the environmental system during Soyuz 18's mission. Salyut 4 was deorbited February 2, 1977, and re-entered the Earth's atmosphere on February 3.

Kosmos-557

Kosmos 557 (Russian: ?????? 557 meaning Cosmos 557) was the designation given to DOS-3, the third space station in the Salyut program. It was originally intended to be launched as Salyut-3, but due to its failure to achieve orbit on May 11, 1973, three days before the launch of Skylab, it was renamed Kosmos-557.
Due to errors in the flight control system while out of the range of ground control, the station fired its attitude thruster until it consumed all of its attitude control fuel and became uncontrollable before raising its orbit to the desired altitude. Since the spacecraft was already in orbit and had been registered by Western radar, the Soviets disguised the launch as "Kosmos 557" and quietly allowed it to reenter Earth's atmosphere and burn up a week later. It was revealed to have been a Salyut station only much later