  Exclusive and kinematically complete high-statistics measurements of the
double pionic fusion reaction $dd \to ^4$He$\pi^0\pi^0$ have been performed in
the energy range 0.8 - 1.4 GeV covering thus the region of the ABC effect,
which denotes a pronounced low-mass enhancement in the $\pi\pi$-invariant mass
spectrum. The experiments were carried out with the WASA detector setup at
COSY. Similar to the observation in the basic $pn \to d \pi^0\pi^0$ reaction,
the data reveal a correlation between the ABC effect and a resonance-like
energy dependence in the total cross section. The maximum occurs at m=2.37 GeV
+ 2$m_N$, i.e. at the same position as in the basic reaction. The observed
resonance width $\Gamma \approx$ 160 MeV can be understood from broadening due
to Fermi motion of the nucleons in initial and final nuclei together with
collision damping. Differential cross sections are described equally well by
the hypothesis of a $pn$ resonance formation during the reaction process.
