A four-level atomic medium driven by probe and control fields is used to control and modify linear and nonlinear absorption and gain. The normal and anomalous dispersions as well as group index are also controlled by the strength and parameters of driving fields. The angular momentum l of the redefined Rabi frequencies of the control fields intensify the number of absorption and gain peaks according to \(2l^2\) condition. Normal dispersion is observed in the region of gain while anomalous dispersion is noted in the region of absorption. The negative group index is studied in the anomalous dispersive region and the positive group index is associated with the normal dispersion region. The normal/anomalous dispersive region and positive/negative group index region also increased by the angular momentum l of the control fields according to \(2l^2\) formula. A positive group index of 1000 is calculated, which reduces the group velocity to \(3\times 10^5 \text {m/s}\) . An approximate negative group index of \(-1000\) is calculated, demonstrating superluminality of the group velocity at \(-1/3\times 10^5 \text {m/s}\) . The modified results of the current study are useful for speeding up computer chips and time cloak technology.
