  Twisted mass lattice QCD (tmQCD), generalised to four Wilson quark flavours,
can be used for the computation of some weak matrix elements related to $\Delta
I=1/2$ transitions. Besides eliminating unphysical zero modes, tmQCD may
alleviate long-standing renormalisation problems of the four-quark operators
which contribute to CP-conserving $K\to\pi$ transitions. With an active charm
quark, the renormalisation of the $K\to\pi$ matrix elements requires at most
the subtraction of a linearly divergent counterterm. Furthermore, in the
(partially) quenched approximation the twist angles can be chosen so that only
a finite counterterm needs to be subtracted.
