How to create audit fields on a table in Postgresql

I need to create a Postgresql table that contains 4 'audit' fields:

• User ID Created
• User Id Modified
• Date Created
• Date Modified

These fields should be populated through the use of defaults and / or triggers.  For example, it is fine to use default values for all 4 fields when a record is created, but after creation, the "user id created" and "date created" fields should never change.  On the other hand, I want the "user id modified" and "date modified" to change every time that the record gets updated.

I am looking for someone to help me complete the following "create table" statement:

create table test_table (id serial
                       , name character varying
                       , user_id_created
                       , user_id_modified 
                       , datetime_created 
                       , datetime_modified
                       )