Catalyzes the reduction of crotonobetainyl-CoA to gamma-butyrobetainyl-CoA. gamma-butyrobetainyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = crotonobetainyl-CoA + reduced [electron-transfer flavoprotein] Amine and polyamine metabolism; carnitine metabolism. Homotetramer. Belongs to the acyl-CoA dehydrogenase family.