This paper deals with the absorption spectra of a weak probe in a four-level Doppler-broadened system driven bythree coherent fields. The main aim is to extend earlier studies of the spontaneous emission spectrum and to presenta comprehensive survey of the spectral features of this system. In addition to a derivation of exact formulae for thespectra, we give an explanation with the help of an appropriate set of dressed atomic states. We also get a deeperinsight into the physical origin of gain in view of the existence of a population inversion between the levels of the lasingtransition. Finally, we explore the effect of Doppler broadening on the absorption profile of the weak probe.