If you need to print sales invoices with more than two decimals on the quantity or unit price, you just need to customize the collection of print templates you are using.
Go to Sales and from the Configuration submenu click on Design & Layout. Locate the template collection you wish to change and click on the far right icon Templates.
You will then see the list of documents relating to the template collection, click on the fare right icon Layouts against invoice...
Click on the option Columns
From the list of columns, locate the column you wish to change and click on the edit icon far right. You can modify the number of decimals to be printed. If necessary, it is possible to increase the width of the column by changing the value on the W (width) (mm) column.
Please bear in mind that if you increase the width of a column you will need to reduce the width of another by the same in order for the columns to appear correctly spaced on your invoice.