In a certain code, RICH is written as 4271 and SNOW is written as 8396. How is COIN written in that code?

This is a direct letter-to-digit substitution cipher. We extract the digit mapping directly from the provided words:
* From RICH (4271): R = 4, I = 2, C = 7, H = 1
* From SNOW (8396): S = 8, N = 3, O = 9, W = 6

Now, substitute the values for the word COIN:

* C = 7
* O = 9
* I = 2
* N = 3

Therefore, COIN is coded as 7923.