The No. 4 Electronic Switching System (4ESS) is a class 4 telephone electronic switching system that was the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in January 1976, to replace the 4A crossbar switch.^[1] The last of the 145 systems in the AT&T network was installed in 1999 in Atlanta. Approximately half of the switches were manufactured in Lisle, Illinois, and the other half in Oklahoma City, Oklahoma. At the time of the Bell System divestiture, most of the 4ESS switches became assets of AT&T as part of the long-distance network, while others remained in the RBOC networks. Over 140 4ESS switches remained in service in the United States in 2007.

This article includes a list of general references, but it lacks sufficient corresponding inline citations. (February 2012)

The 4ESS Switch is often considered as a switching network and its controlling processor. The major functional equipment areas include:

• 1B processor, the 4ESS primary controlling processor
• 3B computer and associated attached processor system, which provides disk storage and additional functions
• CNI ring, a group of peripheral processors serially interconnected with each other in a dual ring configuration, and a 3B20D processor that functions in a distributed input/output processing architecture.
• Terminal equipment provides the interface between metallic trunk facilities in the toll network and the 4ESS Switch. Two-wire and 4-wire voice-frequency trunks terminating at metallic terminal facilities and equipment which provides an interface between digital carrier equipment and the 4ESS switching network.
• The switching network contains the equipment which actually switches pulse code modulated data from one trunk or service circuit to another. The switching network also contains timing equipment that generates the precise timing signals required to switch traffic. The switching network is also used to connect test equipment, announcements, and various tones to trunks when required.

4ESS development began circa 1970, mainly in Naperville, Illinois under the direction of Henry Earle Vaughan. AT&T Long Distance was the primary customer for the switch. Driving development from the customer's perspective was AT&T VP Billy Oliver.^[4] Previous tandem switching systems, primarily the No. 4 Crossbar switch, used analog voice signaling. The decision to switch in a digital voice format was controversial at the time, both from a technical and economic viewpoint. Nevertheless, visionaries such as Vaughn and Oliver recognized that the network would eventually become digital, requiring digital switching technologies.

The last 4ESS was installed in suburban Atlanta, GA in 1999 as a toll tandem for AT&T. At the peak of the product's life time in 1999, AT&T employed 145 4ESS switches in its long-haul network, and several were owned by various Regional Bell Operating Companies (RBOCs).

AT&T replaced or supplemented the 4ESS toll tandem switches with 5ESS switches, which featured an advanced design, and are used as edge switches in the network. Most RBOCs who used 4ESS tandems have replaced them with Class 5 systems of other manufacturers, e.g., Nortel. As of 2014, AT&T operates and maintains approximately one hundred 4ESS switches in the public switched telephone network.

The Nokia N4E-N1B (New 4ESS) is the ATCA-based next-generation toll switch for AT&T. The N4E-N1B includes the 4E APS and 4ESS software, but replaces the 1B processor and the peripheral units which run in emulated environments on an ATCA blade or commercial off-the-shelf servers.^[5] The N4E-N1B is based on the Alcatel-Lucent (now Nokia) gateway platform (7520 Media Gateway (MGW)), 1310 Operations and Management Console – Plus (OMC-P) and the 5400 Linux Control Platform (LCP) and includes other elements such as MRV console terminal servers.

Starting in the late 2010s and continuing in the early 2020s, AT&T is replacing older 4ESS switches with N4E-N1B switches, and is also adding new N4E-N1B switches in places where there was no 4ESS previously. It is assumed that these new N4E-N1B switches are taking over Class-4 functions that were previously handled by 5ESS switches acting as "edge tandems."

• 1ESS switch
• 5ESS switch
• Telephone exchange
• Bell System Practices