Which levels does the SLSA Build track define in the current specification?

Why this is the answer

The Build track runs L0 (no guarantees) through L3 (hardened build platform, non-forgeable provenance). The four-level scheme with a hermetic, two-party-reviewed Level 4 belongs to the retired v0.1 draft and still circulates widely — treat any source citing Levels 1–4 as out of date. SLSA has no tiered certification scheme and is not pass/fail.