It is said on page 15 Smullyan's A beginner's guide to mathematical logic, that
What do we do if we have more than three unknowns, say, A, B, C, D? Well, we can no longer draw circles, but still the four sets divide I into 16 basics regions, and we can number them in such a way that :
A = (1,2,3,4,5,6,7,8)
B = (188.8.131.52.184.108.40.206)
C = (1,2,5,6,9,10,13,14)
D = (1,3,5,7,9,11,13,15)
Why does he say we can no longer draw circles? I did and there are less regions than he has - I got only seven per circle? This would give only 14 regions total in I.