Given that all four friends want to contribute to the daily operation of the business and want the ability to make business decisions, the most appropriate business structure would be:
a general partnership
In a general partnership, all partners can actively participate in managing the business and share in the profits and losses. This structure also allows for flexibility in decision-making among the partners. A sole proprietorship would not be suitable as it is run by a single individual, and a corporation would involve more formalities and a different management structure. A limited partnership would restrict some partners from participating in the business operations. Therefore, a general partnership best fits their needs.