How can we say the neme of this compound as 4-aldo-3-hydroxy-but-2-enoic acid

dear student,In case of multiple functional groups, always start numbering the parent chain from the end of highest priority functional group, -COOH, -COCN,-COOR etc if present, numbering will start from this end itself.the highest priority group will be kept as suffix added to the parent chain, butane(4 carbons parent chain) + oic acid(-COOH) , and all other functional group substituents , for example -COH, -OH, in this case are kept as prefix, in order of their alphabetical order, lastly taking in consideration the double or triple bond present, add it to the parent chain- but-2-enoic acidAldo is written because we are taking the entire -CHO group after C-4 as substituent here.