The starting material was not symmetric, it was CH3OOCCH2CH2CH2OOCCH3, the acetate of methyl 4-hydroxybutanoate. Methoxide would attack each carbonyl. Loss of methoxide from the methyl ester will return a methyl ester. Attack of the acetate carbonyl would give methoxide or the alkoxide of the desired product. With a large amount of alcohol, methoxide would give the desired product through ester exchange.