Layer 4 defines the quality and nature of data delivery and provides for end-to-end integrity controls such as flow control, retransmission etc. These switches control traffic through the use of policies or filters using Layer 4 or transport layer protocol port IDs to differentiate and act upon application-specific traffic. This layer contains application services and protocol formation (i.e., HTTP for Web traffic, FTP for file transfers, POP3 for e-mail servers, Telnet for terminal sessions, SSL for secure communications, etc.) within each packet. Layer 4 makes switching decisions based on the information held in the transport layer, which allows QoS and security controls to be based on either the data or control layers or a combination of the two.
Based upon functionality you can have different switches like Web load balancers, server load balancers and normal switches doing access control or QoS. The same switch can do all these tasks.