Layer 4 of the Open Systems Interconnection (OSI) model is the Transport Layer. This layer is responsible for providing reliable, end-to-end communication services for applications. It segments data from the upper layers, establishes and terminates connections, and handles error control and flow control to ensure data is transferred completely and in the correct order. The two most common protocols at this layer are the Transmission Control Protocol (TCP), which is connection-oriented and reliable, and the User Datagram Protocol (UDP), which is connectionless and offers faster but less reliable data transfer.

A. The data link layer is Layer 2, which manages node-to-node data transfer between two directly connected nodes and handles physical addressing.
C. The network layer is Layer 3, which is responsible for logical addressing (e.g., IP addresses) and routing packets across multiple networks.
D. The presentation layer is Layer 6, which translates, encrypts, and compresses data to a format acceptable for the application layer.

1. International Organization for Standardization. (1994). ISO/IEC 7498-1:1994 Information technology — Open Systems Interconnection — Basic Reference Model: The Basic Model. Section 7.4, "Transport layer," pp. 29-35. This standard formally defines the seven layers of the OSI model, explicitly identifying Layer 4 as the Transport layer.

2. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. Chapter 1, Section 1.5.1, "The OSI model," provides a clear description of the seven layers, stating, "The transport layer (layer 4) transports application-layer messages between application endpoints."

3. Balakrishnan, H., & Terman, C. (2012). 6.02 Introduction to EECS II: Digital Communication Systems, Fall 2012. Massachusetts Institute of Technology: MIT OpenCourseWare. Lecture 1 Notes, "Introduction and The Five-Layer Internet Model," p. 4. The lecture notes map the Internet model to the OSI model, identifying the Transport Layer as Layer 4.