  We develop a strategy for the non-perturbative determination of the
O(a)-improvement coefficient c_sw for Wilson fermions with massive sea quarks.
The improvement condition is defined via the PCAC relation in the Schr\"odinger
functional. It is imposed along a line of constant physics designed to be close
to the correct mass of the charm quark. The numerical work uses the tree-level
improved L\"uscher-Weisz gauge action in N_f=3+1 Lattice QCD.
