  We present first results for the pseudoscalar decay constants $f_K$, $f_D$
and $f_{D_s}$ from lattice QCD with N_f=2+1+1 flavours of dynamical quarks. The
lattice simulations have been performed by the European Twisted Mass
collaboration (ETMC) using maximally twisted mass quarks. For the pseudoscalar
decay constants we follow a mixed action approach by using so called
Osterwalder-Seiler fermions in the valence sector for strange and charm quarks.
The data for two values of the lattice spacing and several values of the
up/down quark mass is analysed using chiral perturbation theory.
