First you have to find out how many he paints in 1 week.
Divide 91/7
You should then have 13 as your answer.
Then do 13 times 4 which would equal 52.
4 weeks = 52 portraits
If you want to see if it is right you can add 13 from 52 and once your at 7 you'll have 91.
Hope this helps! :)