  We develop a model to calculate nonlinear polarization in a nondegenerate
four-wave mixing in diamond configuration which includes the effects of
hyperfine structure and Doppler broadening. We verify the model against the
experiment with $5^{2}S_{1/2}$, $5^{2}P_{3/2}$, $5^{2}D_{3/2}$ and
$5^{2}P_{1/2}$ levels of rubidium 85. Treating the multilevel atomic system as
a combination of many four-level systems we are able to express the nonlinear
susceptibility of a thermal ensemble in a low-intensity regime in terms of
Voigt-type profiles and obtain an excellent conformity of theory and experiment
within this complex system. The agreement is also satisfactory at high
intensity and the analytical model correctly predicts the positions and shapes
of resonances. Our results elucidate the physics of coherent interaction of
light with atoms involving higher excited levels in vapors at room temperature,
which is used in an increasing range of applications.
