Paragraph 1: This should be a short pleasantry which under normal circumstances should not be related to business, such as enquiring if the recipient had a pleasant weekend.
Paragraph 2: Here, as usual for a business letter, you will use one or two paragraphs to convey the message of your letter. Be sure to be concise and to the point.
Paragraph 3/4: Conclusion. This should go over the points you have made in the prior paragraph(s) and thank the reader for his/her time, but you can also mention that you are looking forward to seeing them, or insert another similar nicety to give the letter a more friendly feel.