What is the chemical formula for Tri sulfur Pent oxide?

SO3

Tetra means four, genius. SO3 is Sulphur Trioxide.

Sulphur Tetroxide exists as an anion [SO4]2- More commonly referred to as a sulphate.

Sulphur Tetroxide is not sulphate, sulphate is an ion while Sulphur Tetroxide is not

just like sulphur trioxide (SO3) is a neutral molecule whereas sulphite (SO3)2- is an ion...... so yes, sulfur tetraoxide is SO4, nothing more nothing less