  We examine the ground state and the excitations of an one-dimensional
Heisenberg spin 1/2 antiferromagnet with alternating dimers and four-spin
plaquettes (dimer-plaquette chain). The properties of the system depend on the
competing dimer and plaquette bonds. Several exact, exact-numerical and
perturbational results are presented. We find that the system is gapped for all
parameter values. For the latter one we find an effective S=1 Haldane like
behaviour in the limit of dominating dimer bonds. The model considered is
related to the recently found 1/5-depleted square-lattice Heisenberg system
$CaV_4O_9$.
