A 35mm frame is 1.5 inches long. It is about 1 inch wide, depending on exactly how you do the measurement. Therefore, figure about 1+1/2 square inches per 35mm frame. Multiply by about 40 (36 frames plus a length of leader, rounded off to a convenient number), and you get 60 square inches. That's about 3/4 the area of an 8x10 sheet or four 4x5 sheets, which rounds off to be roughly equivalent in area for the formats.
Actually, the active area 35mm might be slightly smaller than 1+1/2 inches, but likewise a 4x5 is actually smaller than its nominal 4x5 dimensions, so it roughly averages out to equivalent area. Also, the leader is probably not four frames long, but on the other hand it is developed to a much higher density, so it also roughly averages out.
However, in the end a 35mm roll might have slightly less effective area than four 4x5 sheets, so if you are striving for the nth degree of accuracy there might be a slight non-equivalence, with a 35mm roll being slightly smaller than four 4x5 sheets, but probably not to the degree that you calculated.