The measurement units for diamonds are Carats. One Carat equals 200 miligrams.
Addressing a diamond as having 4 grains signifies it is a one carat diamond. The word Carat originates from the word carob. A Carob is a plant (a bean in fact) that grows on trees that prosper in the Mediterranean. In old times had a diamond weighed similarly to a carob bean, its weight was one carob i.e. 1 carat.
Nevertheless in the far east, a place where Carob trees didn’t prosper, rice was the mean to measure the weight of a given diamond. Under that system, had a diamond weighed the same quantity as four grains of rice, then four grains signified 1 carat the way we know it now. Most of the diamond purchases are for diamonds whom weight is about a third of a carat.
Knowing what I just told you, avoid buying diamonds that were arranged or mounted. In case there are at least two diamonds joined in the same piece, the label attached to the jewelry will give the CTW i.e. Carat Total Weight of each diamond in the piece. Hence you should ask the jeweler for exact carat weight of the biggest diamond in the piece to fully understand what you are purchasing.