  We introduce a mixed-action approach based on CLS ensembles, where a valence
$N_f$=2+1+1 Twisted Mass QCD action is combined with the $N_f$=2+1
non-perturbatively O$(a)$-improved Wilson sea sector. We show that for
maximally twisted valence quarks, the automatic O$(a)$-improvement of this
set-up holds up to lattice artifacts coming from sea quark mass effects.
Furthermore, we introduce a three-dimensional Gradient Flow smearing in order
to tame the signal to noise ratio problem.
