  We study the scaling properties of the finite temperature QCD phase
transition, for light quark masses ranging from the heavy quark regime to their
physical values. The lattice results are obtained in the fixed scale approach
from simulations of $N_f=2+1+1$ flavours of Wilson fermions at maximal twist.
We identify an order parameter free from the the linear contributions in mass
due to additive renormalization and regular terms in the Equation of State,
which proves useful for the assessment of the hypothesized universal behaviour.
We find compatibility with the 3D $O(4)$ universality class for the physical
pion mass and temperatures $120$ MeV $ \lesssim T \lesssim 300$ MeV. We discuss
violation of scaling at larger masses and a possible cross-over to mean field
behaviour. The chiral extrapolation $T_0 = 134^{+6}_{-4}$ MeV of the
pseudocritical temperature is robust against predictions of different
universality classes and consistent with its estimate from the $O(4)$ Equation
of State for the physical pion mass.
