Conversion of acetaldehyde to butane-1,4-diol
Cross Aldol and add H+






Oxidise Aldehyde to Carboxylic Acid


use any of the reagents 





3.Perform Arndt–Eistert reaction






Use LiAlH4 for reduction of acid to alcohol




And finally you get 1,4-dihydroxybutane.

Image courtesy Wkipedia, SigmaAldrich