What 29 CFR 1904.4 requires (plain English)
29 CFR 1904.4 is the basic recording requirement that opens OSHA's injury and illness recordkeeping rule (Part 1904). It tells covered employers which cases must be entered on the OSHA 300 Log: each work-related fatality, injury, or illness that is a new case and meets one or more of the general recording criteria in §1904.7 (or the specific-case rules in §§1904.8 through 1904.12 for needlesticks, medical removal, hearing loss, tuberculosis, and the like).
The rule applies to employers required to keep records: those that are not partially exempt by size (10 or fewer employees) or by industry (certain low-hazard NAICS codes) under §§1904.1 and 1904.2. For those employers, 1904.4 is the decision point: run each case through the three-part test, and if all three are met, it is recordable and goes on the Log.
The three parts each have their own section: work-relatedness is defined in §1904.5, "new case" in §1904.6, and the general recording criteria in §1904.7. Section 1904.4 ties them together. Because the 300 Log drives incidence rates, electronic reporting, and enforcement targeting, correct application of 1904.4 is where accurate recordkeeping begins.