Answer
One of the possible answers is:
b, b, d, a, c, b, c, a, d, b, a, c, b, d, d, b, c, b, a, d
(Other answers are possible.)
Work Step by Step
Assign a number from 1 to 4 to a, b, c and d. Use MS Excel to generate 20 numbers in range 1-4.