3PL: Third-party logistics provider – a firm that provides service to its customers of outsourced (or "third party") logistics services for part, or all of their supply chain management functions. (who)
4PL: Fourth-party logistics provider - a general contractor who manages other 3PLs, truckers, forwarders, custom house agents, and others, essentially taking responsibility for a complete logistics process for the customer. (who)
5-whys: An iterative question asking technique used in the “analyze” phase of the Six Sigma DMAIC (define, measure, analyze, improve, control) methodology to explore the cause-and-effect relationships underlying a particular problem. It does not involve any data segmentation, hypothesis testing, regression or other advanced statistical tools, and in many cases can be completed without a data collection plan. The 5 whys typically refers to the practice of asking, five times, why the failure has occurred in order to get to the root cause/causes of the problem. The outcome depends upon the knowledge and persistence of the people involved. In a typical 5 whys exercise, each answer typically forms the next question. For example, when you ask why there was a temperature excursion and answer as because frozen icepacks were used, the second question automatically forms itself as “why frozen icepacks were used?” Also see fishbone diagram.