The term 'Sourdough Unit' is a reference to a type of bread that is made with a sourdough starter. The starter is a mixture of flour, water, and wild yeast that is used to leaven the dough. The starter is kept alive by regularly feeding it with more flour and water. The term 'LOAF' is a 4-letter solution for this crossword puzzle, and it refers to the shape of the bread that is made with the sourdough starter.