When WAP1 locos (few) underwent their POH / Major Overhaul, their important parts like traction motors, transformers, compressor unit, etc are replaced by newer units. Also, auxiliary equipment (if any) are upgraded. Loco is fitted with VCD or SI Units. Then such locos are marked as WAP4 or WAP4E! This is as per my understanding / limited knowledge. Other Members are requested to add / modify this information (for the benefit of all). Thanks!