The width of the rectangle is 26 cm. How much its area will decrease if the length is reduced by 4 cm.

We denote the length of the rectangle by A, and its width by B.

By the condition of the problem, it is known that the width of the rectangle is 26 cm:

B = 26.

Then the area S of the rectangle can be calculated as follows:

S = A * B = 26 * A.

If the length of the rectangle is reduced by 4 cm, then its length will become A1 = A – 4 and the area S2 of the new rectangle:

S2 = A1 * B = (A – 4) * B = 26 * (A – 4).

Therefore, we can calculate the difference in the areas of these rectangles:

S – S2 = 26 * A – 26 * (A – 4) = 26 * A – 26 * A + 104 = 104.

Answer: 104 sq. cm.