This arXiv preprint (not yet peer-reviewed) reports a d=4 QKD experiment that encodes information directly across all four cores of a deployed multicore fiber without mixing in time-bin degrees of freedom. Conducted on an installed campus network at Universidad de Concepción under continuous environmental fluctuations, the setup used superconducting nanowire detectors at 10 dB loss to extract a composable finite-key rate of 6.19 imes10^{-3} bits/pulse—the highest per-pulse figure yet for HD-QKD at comparable attenuation. Earlier field trials relied on hybrid path-plus-time-bin encoding, incurring intrinsic efficiency losses that scale with dimension; this work eliminates that overhead by treating the cores as a complete qudit alphabet. The result matters because multicore fibers are already entering long-haul telecom standards, offering a direct upgrade path for quantum-secure links rather than requiring dedicated dark fiber. Limitations remain: the demonstration is campus-scale, finite-key analysis is used, and environmental noise was present but not quantified over multi-day periods. Related work in Optica (2023) on 7-core fiber QKD and a 2022 PRX Quantum paper on high-dimensional time-bin protocols both showed lower per-pulse rates under similar loss, confirming the efficiency gain. The missing element in prior coverage is recognition that avoiding hybrid encoding also simplifies classical post-processing and detector synchronization, reducing the engineering burden for network operators.