  We present follow-up observations of the K2-133 multi-planet system.
Previously, we announced that K2-133 contained three super-Earths orbiting an
M1.5V host star - with tentative evidence of a fourth outer-planet orbiting at
the edge of the temperate zone. Here we report on the validation of the
presence of the fourth planet, determining a radius of $1.73_{-0.13}^{+0.14}$
R$_{\oplus}$. The four planets span the radius gap of the exoplanet population,
meaning further follow-up would be worthwhile to obtain masses and test
theories of the origin of the gap. In particular, the trend of increasing
planetary radius with decreasing incident flux in the K2-133 system supports
the claim that the gap is caused by photo-evaporation of exoplanet atmospheres.
Finally, we note that K2-133 e orbits on the edge of the stars temperate zone,
and that our radius measurement allows for the possibility that this is a rocky
world. Additional mass measurements are required to confirm or refute this
scenario.
