Mark IV is a drug administered to E-Pandora to enhance their combat capabilities. It is the successor to the Mark III drug. Under its influence, a normal E-Pandora is a match for an original Pandora. The side effects, however, included debilitating pain and eventual partial Nova-fication, as seen in Gina's case.
The drug works by forcing a release of antibody's in proportion to Stigmata activity. However, Mark IV will rapidly lose its stability, losing its boosting effect rapidly, which triggers a 'by the second' aggressive cell division, which results in possible Nova-fication.
When the E-Pandora rebelled, they all took the drug willingly to give them an edge over the Chevalier in their breakout.