These intermediate-range Soviet missiles (about 1,200 miles) were the first to be guided by all-inertial systems.
The Americans found out about them during the Cuban crisis.
They could be fired either from underground silos or from mobile stations and deliver single one-megaton warheads with a CEP of about 1.5 miles.
These 70 feet long missiles constituted the main part of the Soviet strategic force and were the main threat to European countries. They were operational until 1978, when they were gradually superseded by more sophisticated and powerful SS-20s.
Following the 1987 INF Treaty, the last SS-4 was dismantled in 1990.