why four terminal device is called thyristor
The thyristor is a four-layered, three terminal semiconductor device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N.n earlier gas filled tube device called a thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of thyratron and  transistor  that the term thyristor is derived.