AISI/SAE P4 is another low-carbon, carburizing member of the P-series mold steel family, distinguished from grades like P2 by a much higher chromium content — roughly 4.00–5.25%, versus around 1.5% in P2 — combined with molybdenum in the 0.40–1.00% range. Carbon is held very low, at 0.12% maximum, so like the rest of the carburizing P-series, P4 is supplied and machined soft, then case hardened afterward to build a hard, wear-resistant surface over a tough core rather than being hardened all the way through.

The extra chromium in P4 pushes hardenability and case properties further than P2, making it a common choice for mold and die components that need a deeper, more wear-resistant case, along with better resistance to softening at elevated temperature. As with other P-series grades, the soft delivery condition is intentional: it lets toolmakers finish-machine the cavity or insert to final dimensions before the carburizing cycle, minimizing the distortion that would come from machining a fully hardened block.

With carbon at 0.12% or below, P4 machines much like other low-carbon mold steels — cutting forces are modest and tool wear is low in its as-supplied annealed state. The difference from a plain low-carbon grade is the chromium content, which at 4–5.25% is high enough to form a meaningful volume of chromium carbides even at this low carbon level. Those carbides add a bit more abrasive resistance to the cutting edge than a simpler carburizing grade like P2, so tool wear, while still manageable, runs slightly higher than on the lowest-alloy P-series steels.

Chip control follows the same pattern as other soft, low-carbon steels: the material is ductile enough that built-up edge and smearing are a bigger practical concern than tool wear. A sharp, positive-rake edge with good chip evacuation cuts more cleanly than a heavier geometry, and helps avoid the poor surface finish that comes from material dragging across the tool rather than shearing away from it.

Because P4 is always machined before its carburizing treatment, the finish pass on the soft material is the only chance to get dimensions right — the case-hardening step that follows is not intended to correct machining errors. A grade selected for a clean, sharp edge and reliable chip evacuation will generally deliver better results on this material than one chosen purely for maximum wear resistance.