Why do sheeps have four stomachs?

Answer 1

They don't, actually. Sheep have one true stomach with three forestomachs, just like cows/cattle do. And just like with cattle, these multiple chambers help with the digestion of the coarse plant materia that sheep eat. Bacteria, fungi and protozoa numbering in the billions make their home in the rumen to break down cellulose--using an enzyme only they can produce called cellulase--so that the sheep can digest the plant material to the fullest extent.

It is also because many steps are involved in the digestion of plant material that is normally difficult to digest for other animals that are not ruminants. First sheep must swallow plant material, allow it to digest in the rumen, then regurgitate, chew cud, reswallow, allow the bacteria to digest and ferment the digesta again, then remove the excess water in the omasum, then move the chime/digesta into the abomasum (true stomach) to digest proteins and other nutrients that have not been digested and absorbed from either the rumen, omasum or reticulum. The chime then moves into the small intestine where reabsorption of nutrients occur, then to the colon and the cecum for another bout of fermentation, then to the large intestine for reabsorption of water, then to the rectum and anus where it is expelled as feces.