Mass Production of 2021 KMTNet Microlensing Planets I
  We inaugurate a program of ``mass production'' of microlensing planets
discovered in 2021 KMTNet data, with the aim of laying the basis for future
statistical studies. While we ultimately plan to quickly publish all 2021
planets meeting some minimal criteria, the current sample of four was chosen
simply on the basis of having low initial estimates of the planet-host mass
ratio, $q$. It is therefore notable that 2 members of this sample suffer from a
degeneracy in the normalized source radius $\rho$ that arises from different
morphologies of closely spaced caustics. All four planets
[KMT-2021-BLG-(1391,1253,1372,0748)] have well-characterized mass ratios, $q$,
and therefore are suitable for mass-ratio frequency studies. Both of the $\rho$
degeneracies can be resolved by future adaptive optics (AO) observations on 30m
class telescopes. We provide general guidance for such AO observations for all
events in anticipation of the prospect that they will revolutionize the field
of microlensing planets.
