For the next two weeks, head over to the Microsoft Store and you'll find 50% off the Ultimate Edition of Forza Horizon 4. Purchasing the game digitally from the Microsoft Store allows you to play the game both on Xbox 1 and a Windows 10 PC. Cross-play is enabled for both platforms, although Xbox 1 owners need LIVE Gold for online multiplayer while PC (Master Race) players do not.
The Ultimate Edition includes free DLC car packs, some free in-game houses (saving millions of in-game currency), 2 expansion packs (Fortune Island w/ crazy weather and Lego Masters which reviewed less favourably). You may also win more money for races with the included VIP bonus.
A new Top Gear themed update recently was added for free for all versions of the game, so if you're interested now is the perfect time to grab the game, especially as there are no new Forza games due for release this year - a first for a long time. Forza Motorsport 7 development recently ceased with primary focus now shifted to its successor.