Joe-4 was the fifth Soviet nuclear test and demonstrated the use of fusion in a weaponizable design known as the Sloika or “Layer Cake” design.

The device obtained nearly all of its yield from fission and was limited for practical purposes to yields of less than 1Mt. The RDS-6s warhead used a U-235 fissile core surrounded by alternating layers of lithium-6 deuteride spiked with tritium, and a uranium fusion tamper inside a high explosive implosion system.

Though not a true thermonuclear weapon the USSR claimed it was, and in conjunction with the fact that it was air-deliverable caused considerable embarrassment to the US. The US did not successfully test a deliverable thermonuclear bomb until 1954.