Your code has no connection to your verbal description, and I will go with that description, because it actually makes sense.
Perhaps you mean
CREATE TRIGGER ... UPDATE OF a, b, c, d ON tab ...
That will call the trigger function only when one of the named columns has changed. If your worry is enumerating the column names, then my advice is not to be that lazy. You can easily automate that by constructing a CREATE TRIGGER statement from the metadata in information_schema.columns.