  We update our determinations of $f_{D^+}$, $f_{D_s}$, $f_K$, and quark mass
ratios from simulations with four flavors of HISQ dynamical quarks. The
availability of ensembles with light quarks near their physical mass means that
we can extract physical results with only small corrections for valence- and
sea-quark mass mistunings instead of a chiral extrapolation. The adjusted
valence-quark masses and lattice spacings may be determined from an
ensemble-by-ensemble analysis, and the results for the quark mass ratios then
extrapolated to the continuum limit. Our central values of the charmed meson
decay constants, however, come from an alternative analysis, which uses
staggered chiral perturbation theory for the heavy-light mesons, and allows us
to incorporate data at unphysical quark masses where statistical errors are
often smaller. A jackknife analysis propagated through all of these steps takes
account of the correlations among all the quantities used in the analysis.
Systematic errors from the finite spatial size and EM effects are estimated by
varying the parameters in the analysis, and systematic errors from the
assumptions in the continuum extrapolation are estimated from the spread of
values from different extrapolations.
