The primary purpose of EPIC4 is to control the electrostatic precipitator (ESP) by managing the transformer rectifiers (T/Rs) and rapping systems. It ensures efficient particulate collection by charging and depositing particles on collecting plates, which are then removed through rapping. EPIC4 also includes several software packages such as EPOQ (Optimizes emissions), EOPT (Energy Optimization), and PCR (Power Control Rapping) to enhance ESP performance.