Layer 4 is considered to be an important layer although the other layers should not be disregarded because they are important in their own ways. Layer 4 is vital in making sure that recovery and flow control will be done appropriately. The OSI is also known as Open Systems.
This is specifically made in order to show the different processes of network communication. Each layer will have a different mode of function. This can make things easier as each layer can just focus on its own function instead of worrying about the other processes needed to make sure that communication will be clear.
The transport layer (layer 4) defines several functions, including the choice of protocols. The most important layer 4 functions are error recovery and flow control. The transport layer may provide for retransmission, i.e., error recovery, and may use flow control to prevent unnecessary congestion by attempting to send data at a rate that the network can accommodate, or it might not, depending on the choice of protocols.
Multiplexing of incoming data for different flows to applications on the same host is also performed. reordering of the incoming data stream when packets arrive out of order is included. Examples include: tcp, udp, and spx.