The GuestData cookie is a browser cookie that assigns a player's in-game name if they are a guest. It will always be negative, no matter what. The last four digits are the common numbers guests have at the end of them, like "Guest 4182". These numbers are randomly generated every time the guest navigates the site, so it is possible to start off with 4182, click 'Games' and then have 3486.
People were used to be able to change their guest number by editing the cookie using tools such as EditThisCookie. Some YouTubers taught players how to change their guest number to 666 to scare others. However, with the removal of guests in October 2017 and all methods of becoming a guest being removed in November 2018, it is no longer possible to use a legitimate and official method to play as a guest.