The 4K1 prefix was allocated to all USSR Antarctic stations from January, 1973, and this continued until late 1994.
However, 4K1G was used from 1971.

Each USSR base was allocated it's own 4K1x callsign, as follows:

Callsign Base DXCC IOTA QSL Note
4K1A Molodezhnaya Antarctica AN-016 QSL  
4K1B Mirny Antarctica AN-016 QSL  
4K1C Vostok Antarctica AN-016 QSL  
4K1D Novolzarevskaya Antarctica AN-016 QSL  
4K1E Komsomolskaya Antarctica AN-016 Required  
4K1F Bellingshhausen  South Shetland Is AN-010 QSL  
4K1G Liningradskaya  Antarctica AN-016  QSL   
4K1H Russkaya  Antarctica AN-016 QSL  
4K1J Druzhnaya  Antarctica AN-016 QSL  
4K1J Novolazarevskaya Antarctica AN-016  Required  
4K1K Komsomolskaya  Antarctica AN-016 QSL  
4K1L Pionerskaya  Antarctica AN-016 QSL  
4K1R Moldezhnaya  Antarctica AN-016 Required DXNS #743 - QRV for DOSAAF-50
4K1T Unknown Antarctica AN-016 Required DXNS #1527 1992-07-29 Base SP-32? QRV 1992-07-23

Many 4K1xx and 4K1xxx callsigns were also issued. These are listed here .
The first personal callsigns seem to have been issued in 1970.