DXVK is a Direct3D9 to Vulkan layer, and many games can benefit from a performance boost by using it. Before proceeding, be sure that your GPU supports Vulkan.
DXVK's performance boost isn't noticeable on every system but worth a try. DXVK v1.9.4 should be used for GTA IV since it's the most effective one for GTA IV.

Installation:
-1.Download: https://github.com/doitsujin/dxvk/releases (Click on "dxvk-tar.gz")
-2.Open the downloaded file with Winrar/7-Zip and open the x32 folder
-3.If you use the ENB optional rename d3d9.dll from the x32 folder to dxvk.dll and put it in your GTA IV main folder
   If you don't use the ENB optional just put the d3d9.dll from the x32 folder to your GTA IV main folder
-4.Put dxgi.dll from the x32 folder to your GTA IV main folder
-5.Open up the "files" folder and put the "dxvk.conf" file to your GTA IV main folder
   If you use the ENB optional put also the "enbseries.ini" file to your GTA IV main folder
-6.Done

Known issues:
-Could add slightly visible lines at edges on buildings and other models in the distance.
-It could stutter at first launch. Be patient and give the game 3-5 minutes so it will get stable.


Use this link for further information about DXVK in combination with GTA IV: https://www.pcgamingwiki.com/wiki/Grand_Theft_Auto_IV