Assuming this is your only debt and that the amount is not colossal, you have 2 options:-
1. Do nothing. If you have had no contact with the creditor for 4 years then, if you do not have any contact with them for another 2 years (in other words you do not make any payments or acknowledge in any way that you owe the debt) then it becomes statute barred (i.e. after a total of 6 years). This means the creditor can no longer take you to Court to enforce payment of the debt. (The exception is if the creditor has already got a County Court Judgement before the 6 years are up.) The debt still exists after the 6 years but it is then generally possible to settle it for a small fraction of the total amount; or
2. Prepare an income & expenditure statement showing the amount you can afford to pay each month. Then contact the creditor and offer to pay this amount. As you have no paperwork you may find it difficult to get in touch with the correct people, & it is possible the debt has been sold by the original creditor. If you follow this route you will probably have some hassle sorting it out, & may well find the debt is a lot larger than you thought, because interest & other charges will probably have been added.