The series of works ‘Mechanical Stability and Brittleness of Metals and Alloys’ (parts 1–3) presents a systematic review of the problem of failure of metal materials under conditions of their embrittlement due to the stress raisers (SRs). The failure model is based on the concept of fracture as the mechanical instability of a solid due to applied forces. The final, 4-th, part outlines the fundamentals of ideas concerning the specific features of the failure mechanism for metal solids under the non-uniform force fields inside the regions of SR action or during bending of loaded specimens. An original energy–force model of failure of metal materials is considered, in which, due to ductility, a special mechanical property manifests itself, namely, mechanical-stability margin (MSM) (so called ‘break resistance’) B_r The SR destructive force manifests itself in local ‘exhaustion’ of MSM (B_r) within the SR plasticity zone according to the value of the effective stress-raising coefficient α_ef. The critical value of α^N_ef is defined as the SR destructive force, which completely ‘exhausts’ the MSM of metal, B_rc, at the critical brittleness temperature Т_c at the moment of fracture at the yield point of the metal σ_0.2. By definition, α^N_ef = B_rc at Т = Т_c. This work provides examples of application of the specified model parameters, α^N_ef and B_r, to experimental evidence from published works covering a wide range of structural steels (σ_0.2 = 140–2200 МПа, ψ_K = 10–85 %) and various types of CF (cracks, annular notches).