  We present results of lattice QCD simulations with mass-degenerate up and
down and mass-split strange and charm (N_f = 2+1+1) dynamical quarks using
Wilson twisted mass fermions at maximal twist. The tuning of the strange and
charm quark masses is performed at two values of the lattice spacing a~0.078 fm
and a~0.086 fm with lattice sizes ranging from L~1.9 fm to L~2.8 fm. We measure
with high statistical precision the light pseudoscalar mass m_PS and decay
constant f_PS in a range 270 < m_PS < 510 MeV and determine the low energy
parameters f_0, l_3 and l_4 of SU(2) chiral perturbation theory. We use the two
values of the lattice spacing, several lattice sizes as well as different
values of the light, strange and charm quark masses to explore the systematic
effects. A first study of discretisation effects in light-quark observables and
a comparison to N_f=2 results are performed.
