Catalyzes the isomerization of D-erythrulose-4P to D-erythrose-4P. Involved in the degradation pathways of L-threitol, D-threitol and erythritol, that allow M.smegmatis to grow on these compounds as the sole carbon source. D-erythrulose 4-phosphate = D-erythrose 4-phosphate Carbohydrate metabolism; erythritol degradation. Carbohydrate metabolism; D-threitol degradation. Carbohydrate metabolism; L-threitol degradation. Cells lacking this gene are totally unable to grow on L-threitol. Cells lacking both derI1 and derI2 are totally unable to grow on D-threitol or on erythritol. Belongs to the LacAB/RpiB family.