  Using the Schr\"odinger functional (SF) with a single staggered fermion field
we calculate the SF coupling in four-flavour QCD for a wide range of energies
and lattice sizes up to $L/a=16$. Preliminary results for the continuum
extrapolation of the step-scaling function are presented. To reduce cutoff
effects, one-loop ${\rm O}(a)$ improvement has been implemented. Various cross
checks are made possible by the use of two independent sets of lattices with
either $T=L+a$ or $T=L-a$.
