The fourth role that needs to be filled is the The Treasurer or Accountant or Chief Financial Officer.
The Treasurer is concerned with treasure, wealth, capital, assets, hours, stock and debt.
When this role and company are working seemesly together, the job itself is easy.
This role is absolutely essential as the company grows to be able to track and book-keep and, most importantly, plan for the future.
Without them the entire world would grind to a halt, and a startup would only last a few months at best.
The treasurer needs the cooperation of the entire company to execute his job well.
The treasurer needs deadlines, and dates, and limits and ad-spend and may be socially awkward.
You can live without the Treasurer for a very short amount of time but you will want this role filled asap.
Remember, money is the life blood of any organization.