Well, first you will want to find the amount of loads he can do with one box.
Since he can do 1 load with 1/16th of a box, he can do 16 loads with one box. Now we multiply 16 by 4 since he has 4 boxes.
16 * 4 or 16+16+16+16 is equal to 64.
So
Marvin can do 64 loads of laundry with 4 boxes of detergent.
Hope I helped!