L4, or Layer 4, refers to the transport layer in the OSI model, which is a framework used to understand network communication. This layer is responsible for end-to-end communication, ensuring that data is transferred reliably and in the correct order between devices. It manages error detection, correction, and flow control, making it essential for applications that require accurate data transmission. Common protocols that operate at L4 include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP provides a reliable connection-oriented service, while UDP offers a faster, connectionless option. Both protocols play crucial roles in how data is sent and received over the internet.