2,4-dihydroxybutyric acid (2,4-dhb) production method, malate kinase, semialdehyde dehydrogenase malate, dhb dehydrogenase, isolated nucleic acid sequence, chimeric gene, expression vector, host microorganism, use of a methylbutyraldehyde reductase or of a succinic semialdehyde reductase, use of a malate kinase, use of semialdehyde dehydrogenase malate, 4-phospho-malate