Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] can be tailored to achieve a variety of polymeric materials, from hard crystalline plastics to very elastic rubber by varying 4HB monomer composition. In this study, Ralstonia eutropha ATCC 17699 was grown to produce P(3HB-co-4HB) copolymer using a mixture of soybean oil and γ-butyrolactone. The effect of soybean oil and γ-butyrolactone on cell growth was investigated by varying the concentrations in flask cultures. Cell growth was inhibited by high concentrations of γ-butyrolactone over 10 g/L. 4HB fraction was controlled at 0 to 50 mol% by changing the ratio of soybean oil to γ-butyrolactone. Increasing the concentration of γ-butyrolactone in the culture medium resulted in an increase of the 4HB fraction in the copolyester. In 2.5 L fermentor batch culture, cell concentration and 4HB fraction obtained were 5.5 g/L and 13.8 mol%, respectively.