To downgrade, make note of the location for your Steam.exe
Then, paste "steam://nav/console" into your browser. This will open the Steam console.
To downgrade GTA IV, paste "download_depot 12210 12211 4406763129603688303" into the console and hit enter.
To downgrade EFLC, paste "download_depot 12220 12221 9113424693229214791" into the console and hit enter.
If you go to Library > Downloads, you'll see download activity, but no progress. This is intended, just let it download until "Current" hits 0 KB/s.
I have noticed that the download might start slow. If this happens, just wait a few minutes.
Then, the content of the previous version will be in the folders here (in relation to Steam.exe).
steamapps/content/app_12210/app_12210 (GTA IV)
Move the content of these folders into their respective game folder and you should be good to go!