  This paper contains results on stable bundles of rank 2 with space of
sections of dimension 4 on a smooth irreducible projective algebraic curve $C$.
There is a known lower bound on the degree for the existence of such bundles;
the main result of the paper is a geometric criterion for this bound to be
attained. For a general curve $C$ of genus 10, we show that the bound cannot be
attained, but that there exist Petri curves of this genus for which the bound
is sharp. We interpret the main results for various curves and in terms of
Clifford indices and coherent systems.
