The following presumes that you have a unique field by which you are ordering your resultset. For this example, I'll assume that you have some sort of auto-number field in your table called guestbook_entry_id.
Pass an extra variable in the query string (if using a simple link) or a hidden field (if using a form) that contains the last auto_number value displayed. (I'll call this field "last_displayed_number".) On the target script, generate a SQL string with a WHERE clause similar to the following:
"SELECT * FROM guestbook_table WHERE guestbook_entry_id > $last_displayed_number ORDER BY guestbook_entry_id LIMIT 4"
And put the last guestbook_entry_id back into your query string or hidden field.
NOTE: Depending upon your DBMS the "LIMIT" syntax may vary. Consult the manual. You can leave it out, but you'll run a less expensive query with it in place.
