What type of cargo tank is defined by having a pressure of less than 4 PSI and a maximum capacity of 9,000 gallons?

The definition of a cargo tank with a pressure of less than 4 PSI and a maximum capacity of 9,000 gallons aligns perfectly with the characteristics of a non-pressure liquid cargo tank. These tanks are specifically designed to transport liquids that do not require pressure to keep them in a liquid state, making them suitable for a variety of hazardous and non-hazardous materials. The maximum capacity of 9,000 gallons is also a standard specification for this type of tank, which caters to bulk liquid transportation without pressurization.

On the other hand, low pressure chemical cargo tanks are built to withstand pressures greater than what is specified in the question. High pressure cargo tanks, as the name suggests, operate at significantly higher pressures than 4 PSI, often up to 100 PSI or more, allowing them to carry gases and volatile substances. Cryogenic liquid cargo tanks are specialized for transporting items at extremely low temperatures, which does not align with the moderate pressure and capacity outlined in the question. Thus, only the non-pressure liquid cargo tank fits the criteria provided.