  Studying chemical reactions on a state-to-state level tests and improves our
fundamental understanding of chemical processes. For such investigations it is
convenient to make use of ultracold atomic and molecular reactants as they can
be prepared in well defined internal and external quantum states$^{1-4}$. In
general, even cold reactions have many possible final product states$^{5-15}$
and reaction channels are therefore hard to track individually$^{16}$. In
special cases, however, only a single reaction channel is essentially
participating, as observed e.g. in the recombination of two atoms forming a
Feshbach molecule$^{17-19}$ or in atom-Feshbach molecule exchange
reactions$^{20,21}$. Here, we investigate a single-channel reaction of two
Li$_2$-Feshbach molecules where one of the molecules dissociates into two atoms
$2\mathrm{AB}\Rightarrow \mathrm{AB}+\mathrm{A}+\mathrm{B}$. The process is a
prototype for a class of four-body collisions where two reactants produce three
product particles. We measure the collisional dissociation rate constant of
this process as a function of collision energy/ temperature and scattering
length. We confirm an Arrhenius-law dependence on the collision energy, an
$a^4$ power-law dependence on the scattering length $a$ and determine a
universal four body reaction constant.
