  We present new Spitzer transit observations of four K2 transiting
sub-Neptunes: K2-36c, K2-79b, K2-167b, and K2-212b. We derive updated orbital
ephemerides and radii for these planets based on a joint analysis of the
Spitzer, TESS, and K2 photometry. We use the EVEREST pipeline to provide
improved K2 photometry, by detrending instrumental noise and K2's pointing
jitter. We used a pixel level decorrelation method on the Spitzer observations
to reduce instrumental systematic effects. We modeled the effect of possible
blended eclipsing binaries, seeking to validate these planets via the
achromaticity of the transits (K2 versus Spitzer). However, we find that
Spitzer's signal-to-noise ratio for these small planets is insufficient to
validate them via achromaticity. Nevertheless, by jointly fitting radii between
K2 and Spitzer observations, we were able to independently confirm the K2
radius measurements. Due to the long time baseline between the K2 and Spitzer
observations, we were also able to increase the precision of the orbital
periods compared to K2 observations alone. The improvement is a factor of 3 for
K2-36c, and more than an order of magnitude for the remaining planets.
Considering possible JWST observations in 1/2023, previous 1 sigma
uncertainties in transit times for these planets range from 74 to 434 minutes,
but we have reduced them to the range of 8 to 23 minutes.
