  We consider proton decay and $g_\mu - 2$ in flipped SU(5) GUT models. We
first study scenarios in which the soft supersymmetry-breaking parameters are
constrained to be universal at some high scale $M_{in}$ above the standard GUT
scale where the QCD and electroweak SU(2) couplings unify. In this case the
proton lifetime is typically $\gtrsim 10^{36}$~yrs, too long to be detected in
the foreseeable future, and the supersymmetric contribution to $g_\mu - 2$ is
too small to contribute significantly to resolving the discrepancy between the
experimental measurement and data-driven calculations within the Standard
Model. However, we identify a region of the constrained flipped SU(5) parameter
space with large couplings between the 10- and 5-dimensional GUT Higgs
representations where $p \to e^+ \pi^0$ decay may be detectable in the
Hyper-Kamiokande experiment now under construction, though the contribution to
$g_\mu -2$ is still small. A substantial contribution to $g_\mu - 2$ is
possible, however, if the universality constraints on the soft
supersymmetry-breaking masses are relaxed. We find a `quadrifecta' region where
observable proton decay co-exists with a (partial) supersymmetric resolution of
the $g_\mu - 2$ discrepancy and acceptable values of $m_h$ and the relic LSP
density.
