.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.
.txt
bit at the end to .4rpl
(click through the warning message)
You can always repeat this process to set a different editor as the default program to open .4rpl files with.