Enter a number into the sheet, for example, 1234.5678. This number will be displayed in the default number format, with two decimal places. You will see 1234.57 when you confirm the entry. Only the display in the document will be rounded off; internally, the number retains all four decimal places after the decimal point.
To format numbers with decimals:
Set the cursor at the number and choose Format - Cells to start the Format Cells dialogue.
On the Numbers tab you will see a selection of predefined number formats. In the bottom right in the dialogue you will see a preview of how your current number would look if you were to give it a particular format.
If you only want to modify the number of the decimal places displayed, the easiest method is to use the Number Format: Add Decimal Place or Number Format: Delete Decimal Place icons on the Formatting Bar.