Referring to the Transport Layer of the OSI Reference Model. See also OSI Reference Model and Transport Layer.
"Layer 4." YourDictionary, n.d. Web. 24 May 2018. <http://www.yourdictionary.com/layer-4>.
Layer 4. (n.d.). Retrieved May 24th, 2018, from http://www.yourdictionary.com/layer-4
See transport protocol and layer 4 switch.