Did you remember to add 6 inches to all sides? A frame isn't a frame unless it's 6 inches above, 6 inches below, 6inches out to the left, and 6 inches out to the right 😛
Painting = 4x5 = 20 sqft
Frame = (4+.5+.5) (4+.5+.5) = 5x6 = 30 sqft
% = 10/30 = 33%
If you only added 6 inches to the length and width you'd get
Frame = 4.5 x 5.5 = 24.75
% = 4.75/30 = 20ish%