The four-loop QCD corrections to the electroweak $\rho$-parameter
arising from top and bottom quark loops are computed. Specifically we
evaluate the missing ``non-singlet'' piece. Using algebraic methods
the amplitude is reduced to a set of around 50 new master integrals
which are calculated with various analytical and numerical
methods. Inclusion of the newly completed term halves the final value
of the four-loop correction for the minimally renormalized top-quark
mass. The predictions for the shift of the weak mixing angle and the
W-boson mass is thus stabilized.
