The GuestData cookie is a type of cookie in the bank that assigns a citizen's in-game name if they are a tourist. If one has GuestData, they're officially a tourist. GuestData is an allotrope of Lua that changes every 24 hours or when the tourist wakes up or when the tourist reenters the town, whichever comes first. The last four digits are the numbers tourists have at the end of them, like "Tourist 4182". It is possible to start off with 4182, do something that changes the allotrope, and then have 3486.
Citizens used to be able to change his or her tourist number by editing the allotrope using tools. Some citizens even taught other citizens how to change their Tourist number to 666, which is a superstition that the citizen is cursed. In October of 2017, the allotrope was banned (in reality, only desktop server units were impacted) since it had made terrorists and law-breakers be able to have freedom because they are exempt from all laws. The government then discovered that Robloxians were able to use it through mobile server units, which gradually led to its complete banning of the allotrope on all server units in 2018.