If one argues that missiles are essentially the same just because they share the same diameter implying the same boost phase propulsion then by that logic, one would also have to claim that an F-18E/F, F-20, Turkish Hurjet trainer are all same fighter jet simply because they all use the same Turbofan, which is clearly absurd. Or essentially all the liquid fuel missiles and SLVs in the world are all basically just Scuds because they share same boost phase propulsion so rest of the stages, guidance, electronics, manuvering etc does not matter. You probably opened its wiki page and saw the Hwasong's name and thought it must be the same missile.
Missiles are differentiated by far more than their propulsion stage: guidance systems, flight trajectories, apogee, midcourse and terminal-phase parameters, role all define a missile not just boost phase propulsion. A radially controlled RCS exo and endo-atmospheric maneuvering PBV designed to deploy submunitions or suspected MIRV at IRBM ranges makes K-4 massively different from its own MaRVed Khorramshahr predecessor (K-3) and its prototype K-1 let alone Hwasong-10 which never got produced. Forget anything, IISS itself believes that its probably the only actually produced and deployed descendant of Russian R-27 SLBM. Rest of the in-between prototypes or tech demonstrators like BM-25, H-10, K-1, K-2 etc are all insignificant comparisions.