Here are the Forza Horizon 4 System Requirements (Minimum)
CPU: Intel i3-4170 @ 3.7Ghz OR Intel i5 750 @ 2.67Ghz
CPU SPEED: Info
RAM: 8 GB
VIDEO CARD: NVidia 650TI OR NVidia GT 740 OR AMD R7 250x
DEDICATED VIDEO RAM: 2 GB
PIXEL SHADER: 5.0
VERTEX SHADER: 5.0
OS: Windows 10 version 15063.0 or higher

Forza Horizon 4 Recommended Requirements
CPU: Intel i7-3820 @ 3.6Ghz
CPU SPEED: Info
RAM: 12 GB
VIDEO CARD: NVidia GTX 970 OR NVidia GTX 1060 3GB OR AMD R9 290x OR AMD RX 470
DEDICATED VIDEO RAM: 4 GB
PIXEL SHADER: 5.1
VERTEX SHADER: 5.1
OS: Windows 10 version 15063.0 or higher