  The difference in stellar structure above and below spectral type ~M4 is
expected to be a very important one, connected directly or indirectly to a
variety of observational phenomena in cool stars---such as rotation, activity,
magnetic field generation and topology, timescales for evolution of these, and
even the basic mass-radius relationship. In this Cool Stars XVI Splinter
Session, we aimed to use the M4 transition as an opportunity for discussion
about the interiors of low-mass stars and the mechanisms which determine their
fundamental properties. By the conclusion of the session, several key points
were elucidated. Although M dwarfs exhibit significant changes across the fully
convective boundary, this "M4 transition" is not observationally sharp or
discrete. Instead, the properties of M dwarfs (i.e. radius, effective
temperature, rotation, activity lifetime, magnetic field strength and topology)
show smooth changes across M3--M6 spectral types. In addition, a wide range of
stellar masses share similar spectral types around the fully convective
transition. There appears to be a second transition at M6--M8 spectral types,
below which there exists a clear dichotomy of magnetic field topologies.
Finally, we used the information and ideas presented in the session to
construct a framework for how the structure of an M dwarf star, born with
specific mass and chemical composition, responds to the presence of its
magnetic field, itself driven by a feedback process that links the star's
rotation, interior structure, and field topology.
