  We study the dimer-dimer scattering length $a_4$ for a two-component Fermi
mixture in which the different fermions have different masses $m\us$ and
$m\ds$. This is made in the framework of the exact field theoretical method. In
the large mass ratio domain the equations are simplified enough to lead to an
analytical solution. In particular we link $a_4$ to the fermion-dimer
scattering length $a_3$ for the same fermions, and obtain the very simple
relation $a_4=a_3/2$. The result $a_4 \simeq a_3/2$ is actually valid whatever
the mass ratio with quite good precision. As a result we find an analytical
expression providing $a_4$ with a fairly good precision for any masses. To
dominant orders for large mass ratio it agrees with the literature. We show
that, in this large mass ratio domain, the dominant processes are the repeated
dimer-dimer Born scatterings, considered earlier by Pieri and Strinati. We
conclude that their approximation, of retaining only these processes, is a
fairly good one whatever the mass ratio.
