Why j, a, t, & s only?

No doubt that every community displays certain characteristics (attributes) but can these characteristics be used to predict the cast of that community. May not be true but I grabbed this connection and correlated as to why only 4-alphabets j, a, t & s were used to refer to a particular community as jats. My intention is to find the origin of word jats and not the origin or the history or the background of jats because sufficient literature is available on such aspects.