The 4-day wave is a quasi-nondispersive feature which occurs near the winter polar stratopause, with eastward propagating zonal wavenumbers 1 through at least 4, all moving with the same phase speed. The feature has been documented in temperature, geopotential height, wind, and ozone. Synoptic temperature plots show the feature as a ``warm pool'' of air rotating eastward with period near four days. Theoretical studies show that barotropic (and sometimes baroclinic) instability of the polar night jet can produce quasi-nondispersive modes similar to the observed 4-day wave. The present paper presents evidence of the 4-day feature in MLS temperature, geopotential height, and ozone data from August and September, 1992 and 1993. Space-time spectral analyses and filteredsynoptic plots of temperature as a function of pressure and longitude reveal a meridional temperature structure consisting of two peaks, one near the stratopause and one in the lower mesosphere, with an out-of-phase relationship between the two peaks. These characteristics match those recently predicted theoretically by Manney and Randel (1993). The 4-day signal is also found in MLS ozone data. Negative regions of quasi-geostrophic potential vorticity gradient are shown to exist during times of 4-day wave growth, consist ent with instability dynamics playing a role in the developing stages of the feature. Spectral plots of quasi-geostrophic potential vorticity derived from MLS geopotential height fields reveal a 4-day signal peaking near the polar stratopause. The three-dimensional structure resembles the potential vorticity ``charge'' concept discussed recently by Bishop and Thorpe (1994).