In layout mode, click on field. Open inspector, data tab, scroll down to bottom (data formatting). There's a format drop down, choose decimal. Below that , check the box 'Fixed number of decimals: ' , type in 4.
In some cases, you need a calculation to produce a rounded value to a specific number of digits. In those cases, you can use the Round function. But whether you use the round function or data formatting to control the number of decimal places is a question to answer carefully. If this value is used by other calculation fields or script steps to calculate yet another value, you can get a different result between using the round function to round the actual value and the unrounded value that is set to display as rounded. And different situations can make either the rounded or the unrounded value the correct value to use in such a calculation.