You are right about there are three compounds (Y-butyrolactone, 4-Hydroxybutyric acid (4HB), 1,4-Butanediol) that can trigger the synthesis of Poly(3HB-co-4HB) in bacteria.
It seems 4HB is the most efficient substrate since it can be directly converted to 4HB-coA by PhaA enzyme and then further metabolized by PhaB/ PhaC/E to Poly(3HB-co-4HB).
Also Y-butyrolactone, and 1,4-Butanediol may need certain enzymes to convert them into 4HB, which cyano may not have such of those enzymes.
Please review paper in attached and let 's discuss together sometime krub N' Plam.
Have a nice day, Tanakarn