Which of the following is/are the correct statement/s regarding letter writing?

(I) Informal letters are letters we usually write to our friends and family.

(II) A complaint letter is written to the concerned authorities when you are not satisfied with some service, or you have a problem that needs to be addressed.

(III) A complimentary close is placed before the body of the message as a way to politely start the correspondence.

(IV) A circular is written on plain paper.