First, we can calculate the number if portraits he can paint in one week.
We can do this by simply dividing the total portraits by the weeks needed.
91 ÷ 7
=13
Now, to calculate the number of portraits he can paint in 4 weeks, just multiply the total number of paintings he can do in a week by 4.
13 x 4
=52
So the answer is 52 portraits.