Error 004 is annoying, but easy to solve. Don’t worry.
If you're trying to patch your client and are continually presented with "League of Legends Error 004," a button that says "OK," and your client restarting, then we have the solution. It's a simple problem with a simple fix.
How to fix Error 004 in League of Legends
League of Legends is full of bugs, but luckily, Error 004 is a pretty straightforward error when it comes to causes. The error is caused by the League of Legends client trying to patch but lacks sufficient storage space on the drive that League of Legends is installed on in order to fully download the patch's contents. Simple and sweet — you don't have enough space to update the game.
How to fix Error 004
This can be solved in a few ways. First is deleting content from the drive you have League of Legends on to clear space. This can be done by using the "Add or Remove Programs" function of the Control Panel on Windows or by dragging the files to the Trash and then using the Clear Trash function on Mac.
The next way, if you have multiple drives and want to maintain all of the other files on the drive you had League of Legends on, is to move drives. This can be done by just finding League of Legends in your File Browser or Finder. Then, select the League of Legends folder and move it to the drive of choice. Note that you will need to create a new Shortcut icon if you want to launch from that new file location from your desktop, apart form that everything should be fine.
That should solve Error 004 if you're running into it without explanation. Good luck and stick around esports.gg for more news and updates.