  We study an inflationary model developed by Kaloper and Sorbo, in which the
inflaton is an axion with a sub-Planckian decay constant, whose potential is
generated by mixing with a topological 4-form field strength. This gives a 4d
construction of "axion monodromy inflation": the axion winds many times over
the course of inflation and draws energy from the 4-form. The classical theory
is equivalent to chaotic inflation with a quadratic inflaton potential. Such
models can produce "high scale" inflation driven by energy densities of the
order of $(10^{16}\ GeV)^4$, which produces primordial gravitational waves
potentially accessible to CMB polarization experiments. We analyze the possible
corrections to this scenario from the standpoint of 4d effective field theory,
identifying the physics which potentially suppresses dangerous corrections to
the slow-roll potential. This yields a constraint relation between the axion
decay constant, the inflaton mass, and the 4-form charge. We show how these
models can evade the fundamental constraints which typically make high-scale
inflation difficult to realize. Specifically, the moduli coupling to the
axion-four-form sector must have masses higher than the inflationary Hubble
scale ($\la\ 10^{14}\ GeV$). There are also constraints from states that become
light due to multiple windings of the axion, as happens in explicit string
theory constructions of this scenario. Further, such models generally have a
quantum-mechanical "tunneling mode" in which the axion jumps between windings,
which must be suppressed. Finally, we outline possible observational
signatures.
