The price of the coat was reduced by 20%.
To calculate the percentage reduction in price, we need to find the difference between the original price of $90 and the reduced price of $72.
The difference is $90 - $72 = $18.
To find the percentage reduction, we divide the difference by the original price and multiply by 100:
Percentage reduction = ($18 / $90) * 100 = 20%.
Therefore, the price of the coat was reduced by 20%. The correct answer choice is 4) 20.