The trader pays his debt in 6 weekly installments. Each week, he pays an amount that is a multiple of the amount he paid in the first week. Specifically, in the nth week, he pays n times the amount he paid in the first week.

In the first week, he paid ₦4. In the second week, he paid twice this amount, or ₦8. In the third week, he paid three times the first week's amount, or ₦12, and so on.

We can represent the total amount he paid as the sum of these weekly payments. This can be calculated as the sum of 4n for n from 1 to 6:

\[\text{Total Debt} = \sum_{n=1}^{6} 4n\]

Evaluating this sum gives:

\[\text{Total Debt} = 4 + 8 + 12 + 16 + 20 + 24 = ₦84\]

So, the total amount of the debt the trader owed was ₦84.