A series of theoretical investigations of the quaternary InAlGaAs material system are carried out. The electronic band structures, the density-of-states, the optical gain spectra and the radiative current density have been investigated based on the Hamiltonian derived using the k.p method. We investigated the dependence of the optical gain and transparent current density on the well width, barrier height, and strain using a numerical approach with high accuracy. By varying the well width, mole fraction in the well material and the Al mole fraction in the barrier the effects of quantum confinement and compressive strain are examined. Furthermore, we demonstrate that a reduction of the well width offers improved modal gain over all radiative current densities. Two different material systems, InAlGaAs on InP and GaAs substrates, strained and unstrained, are examined, respectively. Our results suggest that a suitable combination of well width and barrier height should be selected in improving the TE mode optical gain in InAlGaAs single QW. A very low transparent threshold current density 35 A/cm2 of (formula available in paper) strained quantum well laser is predicted. These theoretical results would be useful for design and further performance improvement of the quaternary InAlGaAs strained QW laser diodes.