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 "catalog" and then have 3486.
People were use to be able to change their guest number by editing the cookie but as of 2016, this doesn't change your guest number anymore.