GTA IV Magic Patcher is a utility that allows you to easily mod Grand Theft Auto IV. If you want to install mods in the game, you need a tool like this. Without a patch, GTA IV won't run if files have been altered, but GTA IV Magic Patcher tricks the game into thinking the files are normal, allowing you to change whatever you want.
To use GTA IV Magic Patcher, simply unzip the file and backup the existing filelist.pak in your Rockstar Games directory. Then replace the original file with the filelist.pak in this archive. This will enable you to install mods without any problem. While it would be more convenient if the process was automated, using GTA IV Magic Patcher doesn't require too much effort.
Overall, GTA IV Magic Patcher is a simple and effective way to start having more fun with Grand Theft Auto IV.