  We present a determination of the gradient flow scales $w_0$, $\sqrt{t_0}$
and $t_0/w_0$ in isosymmetric QCD, making use of the gauge ensembles produced
by the Extended Twisted Mass Collaboration (ETMC) with $N_f=2+1+1$ flavours of
Wilson-clover twisted-mass quarks including configurations close to the
physical point for all dynamical flavours. The simulations are carried out at
three values of the lattice spacing and the scale is set through the PDG value
of the pion decay constant, yielding $w_0=0.17383(63)$ fm,
$\sqrt{t_0}=0.14436(61)$ fm and $t_0/w_0=0.11969(62)$ fm. Finally, fixing the
kaon mass to its isosymmetric value, we determine the ratio of the kaon and
pion leptonic decay constants to be $f_K/f_{\pi}=1.1995(44)$.
