You want a polished, stable, “just works” experience. Official co-op does not exist, and this mod feels like a duct-taped experiment.
Play on a dedicated server (if available) or with a very good host. Save often – and keep backups. Lower your expectations from “polished co-op” to “glitchy sleepover party game.” Would you like a step-by-step installation guide for the mod instead? stranded deep 4 player mod