The WAP-4 is one of the most successful locomotives of Indian Railways[peacock prose] serving passenger trains for over 29 years.

The WAP-4 class was developed after its predecessor, the WAP-1, was found inadequate to haul the longer, heavier express trains (24-26 coaches) that were becoming the mainstay of the Indian Railways network.

So, these traction motors were adopted on to WAP-1s Flex-icoil Mark I fabricated bogies, and with a new indigenously designed 5400 kVA transformer and silicon rectifiers.

To accommodate the heavier Hitachi motors without increasing its 112t overall weight, the WAP-4 was made substantially lighter by the widespread use of aluminum materials in construction.

Newer locos also feature larger windshields, more spacious driver cabin with bucket type seats and ergonomic controls.

From 2000, newer versions with many WAP-5 design cues like square type windscreens ,twin-beam headlights, speed recorders and some changes to the control electronics had been rolled out recently.