Dubbletrubble:
What you describe sounds very good, but you always have a probability that "things go wrong" when you deal with other persons in a non well established bussiness situation.
You already mention that there are 3 men in that bussiness and you are the 4th.
People are good until they don´t If the bussiness prospers and more money are generated, the "good persons" could change and they can "remove you" from the bussiness easy just because you are in that place only by "mouth agreement"
Four persons "assosiated" in a small bussiness are a lot of "heads" and sooner or latter that will bring discrepancies between them.
I do not want to tell this to disapoint you, but to have "precaution"; at least talk about that, if for any reason you decided to leave, all the tools tha belong to you can be removed from the store in the moment you need.
Please do not misunderstand my coments, I wish you all the luck and go ahead, but try to put your situation as clear as posible to avoid troubles in the future.
Regards,