Concept:
Partition of set into equal groups
Step 1: Total elements = 20, each set has 5 elements. Step 2: Arrange all elements: 20!
Step 3: Divide by $(5!)^4$ (internal arrangements of each set). Step 4: Since sets are distinct (A, B, C, D), no further division. Conclusion:
Number of ways = $\dfrac{20!}{(5!)^4}$