Poly(3-hydroxybutyrate-co-4-hydroxybutyrate)s (P(3HB-co-4HB)s) with a wide range of 4HB contents were biosynthesized, and their crystalline states were studied by differential scanning calorimetry (d.s.c.) and compared with those of the cold-stretched copolyesters of poly(3HB-co-3-hydroxyvalerate) (P(3HB-co-3HV)). A structural study was also carried out by the use of molecular mechanics calculations. It was demonstrated that P(3HB-co-4HB) cannot form isomorphic crystals because of the extent of the structural difference between the 3HB and 4HB comonomer components.