Go to your steam library and right click on Sonic 4 Ep 1. Then click properties. Navigate to the Local Files section and click 'verify integrity of game files'.

Go to your steam library and right-click on Sonic 4 Ep 1. Then, click 'browse local files'. You should now see three .exe files: Sonic_vis, Sonic_xp and SonicLauncher. These represent the Operating systems that they are built for: Windows Vista, Windows XP and Windows 7, 8, 10 and 11.
Repeat these steps for all three .exe files:

1. Right-click on the executable file.
   
2. Click 'Properties'.
   
3. In the new window that has opened up, click 'Compatibility'.
   
4. Check the 'Run in Compatibility Mode' box.
   
5. In the drop-down menu underneath the Checkbox, select Windows XP Service Pack 3.
   
6. Click 'Apply'.
   
7. Click 'OK'.

The '4 horsemen' are my nickname for the for .dll (Digital-link library) files that DO NOT INSTALL WITH THE GAME FOR SOME REASON. If you try to open the vista or XP versions of the launcher, you will see that these four .dll files are 'not found'.

Go to these four links (don't worry they are completely safe):

• https://www.dll-files.com/xinput1_3.dll.html
  
• https://www.dll-files.com/msvcr100.dll.html
  
• https://www.dll-files.com/msvcp100.dll.html
  
• https://www.dll-files.com/d3dx9_41.dll.html

Download the Windows 32 versions of these files (not the Windows 64 versions as they will give you error 0xc000007b when you launch the game).
Then, go to your Steam library and right-click on Sonic 4 Ep 1. Then click 'browse local files'.
Go to the .zip files and copy and paste all the .dll files from the 4 .zip files into the Sonic 4 Ep 1 folder.

Now, cross your fingers and launch the game. It should work.