@Yinchi0 I had a friend, an old man who has since passed away. When he was young, he ran a commune in Northern California back in the 60's. He told me that when a new person moved into the commune, he would tell them, "So you are welcome to stay as long as you get along with everyone here, but whenever you decide to move on, take everything you brought with you. Don't expect that you can leave your belongings here and come back for them some other time. Anything that gets left behind goes to the dump or passed on to someone who can use it. If you leave it, it's not yours anymore."
I've had a few guests leave a shirt or something, and I have no issue with letting them know. Some say to just keep it or pass it on, some would like it back and I find a way to do that. But 4 years? What a brat.