Need Help with a Simple IF THEN Statement

I am trying to do a simple if-then calculation such as:

IF [Column A] = 4 THEN [Column B] * -1

ELSE [Column A]

I want to take a price column and make it negative for returns. I have looked at the Rule Engine, Column Expressions, Java Snippet, Column Lag, and other nodes, but it is still not clear how to make this simple calculation. Can some please offer any suggestions? Thank you.