In aldotetrosas there are two asymmetric carbons. As in the rest of the monosaccharides, the D or L configuration is determined taking as a reference the asymmetric carbon furthest from the carbonyl group, which in the aldotetrose is carbon 3.
As has been said, two carbohydrates are called epimers that only differ in the position of the hydroxyl group (OH) of an asymmetric carbon. Thus, for example, L-erythrose and L-treose are epimers. They are different substances and, therefore, with different properties.
Ketotetrosses have only one asymmetric carbon, and it is located at carbon 3.