This paper develops the observability layer of VBRC. It does not generate new retained laws; instead, it studies how already licensed retained summaries become observable under a declared protocol. Under the Internal Invisibility Principle, unread content cannot enter observable physics as raw data. It contributes only through a hidden-to-retained summary channel, while observation is described by a readout map, a measure, and representation-dependent update data. The main result is a deterministic-versus-probabilistic dichotomy. If the retained deterministic update factors through the readout, the observed update is deterministic. If it does not, then the next observed output is governed by a non-Dirac conditional law on a positive-measure set of readouts. Probability is therefore not introduced as an independent axiom. It arises from disintegration of the protocol measure along the readout map, as the protocol-level expression of partial observation under IIP. From this structure the paper obtains pushforward probability, fiber-weight observational entropy, event and update rules by conditionalization, record accumulation, and operational quotient/gauge equivalence. Concrete physical sectors are left to later implementations. Part IV therefore provides the general protocol calculus by which retained VBRC laws are read as observable probabilities, events, entropies, records, and operational symmetries.
Yi (Fri,) studied this question.