This is a good question, and the answer is not so easy :)
The less content the more design effort must be done. (less is more) and don’t worry about white spaces because spaces is quite nice when you design it in a good manner.
My answer will be base over three main axes
First the content, second the layout design, third the way you will fold the letter.
I will start with the third point.
The way you will fold your letter will determine on which folded side will be your content. The folded lines is somehow will be a part of your design and it could enrich the content you are presenting by categorizing the content in each folded area.
Most probably you will have two choice folding your letter, the first one is the tri fold letter or the Z fold letter …your choice will be the basement of your letter layout design.
Why we consider the fold? Because your letter will be read first when it is folded the time it will be pulled out from its envelop. Oh by the way the envelop is another factor but we will not go through it.
Let’s say you will fold it Z fold letter, that’s mean you have three main areas must be considered to distribute your content. The cover, the inside middle page and the inside lower page.
ps. some Z fold make the cover inside, personally I dont like it.
The cover page should contain the company logo and the company name it may contain the name of The recipient the inside pages may contain the letter text and the lower folded page may contain the sender information and the signature, and if there is a footer it may be appear in that part. Modern letters layout have no footer.
The Second point is the design content issue, where to put what we decided to put in the first point?
Following the folding areas there is another proportion lines we have to consider when deciding where to put your content.
- The paper folded area (it will follow the rule of third) (green
- The Golden section proportion which is the exact A4 proportion (red Lines)
- The paper margin (blue Lines), top and bottom margins may differ according to the ratio determine by the diagonal dotted line as shown hereunder image
- A grid of 4*4 in the working area (orange lines) to distribute the elements vertically
- The diagonal line from the top left to the lower right
From all the above proportions guides I may select the best part to represent my content and not be afraid about any huge white spaces. Respecting propotion may lead to the human eye comfort. The Intersected area is the shaded one which will contain the letter content.
Now comes to the content
Most probably you may need more text to put without touching the main “Thank you wording” the official business letters must have some elements and @JoostS listed some of it, and I will put my two penny, Vertical lines and the watermarks could be an added value in the design and may break the white spaces a little.
The final notes, you may choose between good papers material color and texture will enrich your letter so much, don't use a plain white papers. and of course sing the letter by a ink pen.
the example below is a sample of a letter following all the above rules.