Catalyzes the reversible conversion of 4-hydroxybutyryl-CoA to crotonyl-CoA. The mechanism of the reaction seems to go through three steps: (1) the FAD-dependent oxidation of 4-hydroxybutyryl-CoA to 4-hydroxycrotonyl-CoA; (2) the hydroxyl group is substituted by a hydride derived from the now reduced FAD in an SN2' reaction leading to vinylacetyl-CoA; (3) isomerization to yield crotonyl-CoA. 4-hydroxybutanoyl-CoA = (2E)-butenoyl-CoA + H2O vinylacetyl-CoA = (2E)-butenoyl-CoA Binds 1 FAD per subunit. Binds 1 [4Fe-4S] cluster per subunit. Homotetramer.