  We investigate the vector form factor relevant for the $K_{\ell 3}$
semileptonic decay using maximally twisted-mass fermions with 4 dynamical
flavours ($N_f=2+1+1$). Our simulations feature pion masses ranging from $210$
MeV to approximately $450$ MeV and lattice spacing values as small as $0.06$fm.
Our main result for the vector form factor at zero 4-momentum transfer is
$f_+(0)=0.9683(65)$ where the uncertainty is both statistical and systematic.
By combining our result with the experimental value of $f_+(0)|V_{us}|$ we
obtain $|V_{us}|=0.2234(16)$, which satisfies the unitarity constraint of the
Standard Model at the permille level.
