  We show how a single linearly polarized control field can produce a sharply
tunable group velocity of a weak probe field at resonance in a four-level
atomic configuration of alkali vapors. The dispersion can be switched from
normal to anomalous along with vanishing absorption, just by changing intensity
of the resonant control field. In addition, by allowing different intensities
of the different polarization components of the control field, the anomalous
dispersion can be switched back to the normal. This thereby creates a "valley
of anomaly" in group index variation and offers two sets of control field
intensities, for which the system behaves like a vacuum. The explicit
analytical expressions for the probe coherence are provided along with all
physical explanations. We demonstrate our results in $J = 1/2 \leftrightarrow J
= 1/2$ transition for D_1 lines in alkali atoms, in which one can obtain a
group index as large as $3.2\times10^{8}$ and as negative as $-1.5\times10^{5}$
using a control field with power as low as 0.017 mW/cm$^2$ and 9.56 mW/cm$^2$ .
