  We present results for the form factor $f_+^{K \pi}(0)$, needed to extract
the CKM matrix element $|V_{us}|$ from experimental data on semileptonic $K$
decays, on the HISQ $N_f=2+1+1$ MILC configurations. The HISQ action is also
used for the valence sector. The data set used for our final result includes
three different values of the lattice spacing and data at the physical light
quark masses. We discuss the error budget and how this calculation improves on
our previous determination of $f_+^{K \pi}(0)$ on the asqtad $N_f=2+1$ MILC
configurations.
