.4rpl files are simply normal text files with a different extension, they can be opened with any generic text editor. The community frequently uses Notepad++, but the default Notepad program will also work.

Pressing the button from within the game will always ask you with which program to open the file. This pop up will show up every time you try to open a 4rpl file until you have set a file association (a default program to open this file type with). This particular pop-up does not have the ability to set the file association (for whatever reason), so we'll have to manually do that by creating a .4rpl file ourselves and setting the association through that.

• Make sure you have file name extensions enabled in the Explorer ¹⁾
• Go to your desktop
• Right click and select “New > Text Document” ²⁾
• Give the file any name, but replace the .txt bit at the end to .4rpl (click through the warning message)
• Right click the created file and select “Open With > Choose another app” ³⁾
• Make sure the “Always use this app” checkbox at the bottom is checked and then click the program you want to use. ⁴⁾
• If your preferred program is not in the list click “More apps” to expand the list. If it is not in that list either you'll have to select “Look for another app on this PC” at the very bottom and directly specify the .exe of your program.
• You may now delete the desktop file. Pressing the button from within the game should now open any 4rpl file in the editor specified by you.

You can always repeat this process to set a different editor as the default program to open .4rpl files with.