The GreenFlag Four Stage Approach is a debt collection process that involves 5 contacts with customers over 58 days. It begins with sending an invoice or statement, followed by 4 reminder letters or calls from the client at 30, 37, 44, and 51 days. If no response, a final warning letter is sent at 58 days. After 58 days, the account is sent to a third party for pre-collection demands made every 10 days for up to 120 days. The process aims to systematically contact customers and collect debts within 58 days using diplomatic efforts before progressing to more intensive collection methods.