Catalyzes the isomerization of D-erythrulose-4P to D-erythrose-4P. Involved in the degradation pathways of erythritol and D-threitol, 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. Up-regulated during growth on erythritol, D-threitol or L-threitol relative to growth on glycerol. Cells lacking both derI1 and derI2 are totally unable to grow on D-threitol or on erythritol. Belongs to the LacAB/RpiB family.