  We present a lattice QCD determination of the vector form factor of the kaon
semileptonic decay K -> pi l nu which is relevant for the extraction of the CKM
matrix element |V_{us}| from experimental data. Our result is based on the
gauge configurations produced by the European Twisted Mass Collaboration with
N_f=2+1+1 dynamical fermions. We simulated at three different values of the
lattice spacing and with pion masses as small as 210 MeV. Our preliminary
estimate for the vector form factor at zero 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.
