Catalyzes the isomerization of D-erythrulose-4P to D-erythrose-4P. Involved in the degradation pathway of erythritol, that allows B.abortus to grow on this compound as the sole carbon source. D-erythrulose 4-phosphate = D-erythrose 4-phosphate Carbohydrate metabolism; erythritol degradation. Cells lacking this gene do not grow on erythritol as sole carbon source, in contrast to wild-type. Belongs to the LacAB/RpiB family.