Strictly speaking, the full version of Forza Horizon 4 is . However, there are legitimate ways to experience it or acquire it if you meet certain criteria:
FORZA HORIZON 4 for Xbox One/PC is a digital code with no expiry date. Go to product viewer dialog for this item.
Forza Horizon 4 (United States) (PC / Xbox One / Xbox Series X|S) - Xbox Live - Digital Key