Create a calculation field (with the calculation result set to Number) and use the following calculation:
Max ( field_A ; field_B ; field_C ; field_D)
The calculation field will then display the highest value of the four fields.
If you have multiple fields that hold the same type of information, it may be useful to move those fields into a separate related table and use a portal to list them on your layout. That may or may not be a useful suggestion for you. If you do use a portal you can still use the max function Nick suggests, but now it will look like Max ( RelatedTable::Field ) and will return the maximum of all the values listed in your portal.
Thank you very much.
I solved with Nick's method.
Thank you again.