**Solution:**

To find the difference between the maximum and mean shear stress in the web of an I-joist, we need to consider the distribution of shear stress across the web.

**1. Shear Stress Distribution:**

The shear stress distribution across the web of an I-joist is triangular. The maximum shear stress occurs at the neutral axis, while the mean shear stress is the average shear stress across the entire web.

**2. Calculation of Maximum Shear Stress:**

The maximum shear stress can be calculated using the formula:

σ_max = V * Q / (I * t)

Where,
σ_max = Maximum shear stress
V = Shear force
Q = First moment of area of the web about the neutral axis
I = Moment of inertia of the web about the neutral axis
t = Thickness of the web

Since the shear force is not given in the question, we cannot determine the exact value of the maximum shear stress.

**3. Calculation of Mean Shear Stress:**

The mean shear stress can be calculated using the formula:

σ_mean = V / (b * t)

Where,
σ_mean = Mean shear stress
V = Shear force
b = Width of the web
t = Thickness of the web

Since the width and thickness of the web are not given in the question, we cannot determine the exact value of the mean shear stress.

**4. Difference between Maximum and Mean Shear Stress:**

Since we do not have the exact values of the maximum and mean shear stresses, we cannot calculate their difference accurately. However, we can make a general observation.

In a triangular distribution of shear stress, the maximum shear stress is typically higher than the mean shear stress. Therefore, the difference between the maximum and mean shear stress would be positive.

However, based on the given options, we can make an assumption that the difference between the maximum and mean shear stress is inversely proportional to the moment of inertia (I) about the neutral axis.

Therefore, the correct option would be (4) 1/40I, as it implies that the difference between the maximum and mean shear stress decreases as the moment of inertia increases.

Note: It is important to have the exact values of the shear force, width, and thickness of the web to calculate the maximum and mean shear stresses accurately.