Mega means 1,000,000
Giga means 1,000,000,000
Tera means 1,000,000,000,000
and you are correct is saying that
Tetra means 4
but that doesn't fit with Mega and Giga
FF7 had the Tetra Elemental and FF9 and FF11 have Tetra Master, and FFX2 has several Tetra items, but I don't know what that has to do with anything.