If you execute the flag04 file on the given path, you will get the message "you may not access". At source code, I saw that access is not possible if the word "token" is present in the file name.
So, I created the link without the word "token" at file name. One is a soft link and the other is a hard link. Note that you should input an absolute path when creating a soft link.
And I executed flag04 file. some string appears.
After checking the string, I was able to know that it was the password for the flag04 account. After login into flag04 account, Clear the level with getflag command.