Explanation

If 4 mat-weavers can weave 4 mats in 4 days, then the total number of mats woven by 1 mat-weaver in 4 days is 1.

So, in 4 days, 8 mat-weavers would weave 8 mats, and each mat-weaver would weave 1 mat.

Thus, in the same 4 days, 8 mat-weavers would weave 16 mats.