Tav = 400, ordinal 22, reduced 4, no final form. Spelled in full, תו = 400+6 = 406.
Meaning
Tav is 'mark' or 'cross' — the sign set on the forehead in Ezekiel's vision — and it is the last letter, one of the seven doubles (t/th), so the alphabet ends with a seal. Its 400 is the number of the years of the Egyptian sojourn foretold to Abraham, and emet (truth) ends in tav, which is why the tradition calls the letter the seal of the world.