  We consider collisional properties of weakly bound heteronuclear molecules
(dimers) formed in a two-species mixture of atoms with a large mass difference.
We focus on dimers containing light fermionic atoms as they manifest
collisional stability due to an effective dimer-dimer repulsion originating
from the exchange of the light atoms. In order to solve the dimer-dimer
scattering problem we develop a theoretical approach, which provides a
physically transparent and quantitative description of this four-atom system in
terms of three- and two-body observables. We calculate the elastic scattering
amplitude and the rates of inelastic processes such as the trimer formation and
the relaxation of dimers into deeply bound molecular states. Irrespective of
whether the heavy atoms are bosons or fermions, the inelastic rate can be
significantly lower than the rate of elastic collisions. Moreover, the
measurement of the inelastic rate which is a four-body observable, can be an
efficient and precise tool for determining three-body observables such as the
three-body parameter, positions of Efimov states and their lifetimes.
