Although it was Soviet's fifth nuclear detonation, for some reasonon the west it was dubbed as Joe 4.
I doubt US intelligence would miss an atomic explosion in USSR, so I guess it was something else. Anyway, the device
codenamed RDS-6 was built upon Sakharov's Sloika design. It was not
a true H-Bomb, as most of its yield came from fission. Nevertheless, it demonstrated fusion reaction in a deliverable weapon
sized device. Obviously that bolstered up US determination to create a deliverable thermonuclear weapon and took nuclear arms
race one step further.
RDS-6 employed both, fision and fusion rections. Fissile material - U235, which was layered with fusion fuel - Li6, and that in turn was mixed with Tritium. The practical yield limit for this type of device was below 1mgt. It was not widely deployed, as more successfull Teller-Ulam design appeared soon.