What is the difference between two-pipe and four-pipe HVAC systems?

Two-pipe HVAC systems use a single pipe for both chilled water supply and return, whereas four-pipe systems use separate pipes for chilled water supply and return, as well as for hot water supply and return. Four-pipe systems provide more flexibility and design options, but are typically more complex and expensive than two-pipe systems. Two-pipe systems are often used in smaller buildings or applications with simpler heating and cooling requirements.