What is the formula for tin (IV) chromate?

Sn(CrO4)2

Tin 4 means that the tin is a cation with a +4 charge.

Chromate is a polyatomic ion with the formula (CrO4)-2. Since the tin ion has a +4 charge in this case, and the chromate ion has a -2 charge, there is a 1:2 ratio of tin ions to chromate ions.