If North Korea has actually conducted a hydrogen bomb test this may the first of several tests to prove reliability.
But "hydrogen bomb" need not mean the very powerful two stage hydrogen bomb. Hydrogen bombs can include far less powerful fusion boosted fission devices. Such a fusion boosted fission device may have been detonated by Pakistan in 1998 – then design plans and some parts passed to North Korea via the proliferation of A Q Khan with the help/knowledge of Pakistan’s military and political bosses.
With an estimated yield of only 9 kilotons the 4th test probably was not aHydrogen Bomb (diagram above). A convincing Hydrogen Bomb test would more likely be around
100+ kilotons. Above is abasic design of a two stage Hydrogen Bomb (using the Teller–Ulam method). Radiation from a circular primary fission bomb compresses a secondary section containing both fission and fusion fuel. The compressed secondary is heated from within by a second fission explosion.
Aficionados of nuclear weapons might assess that North Korea has just announced improvements in the three important areas (device, nuclear explosive and weapon delivery means) of a nuclear program.