Hiding a field based on another field in the same Tab

I'm trying to hide a field below another field once one of the four options is chosen but they are in the same tab. I know how to hide a complete tab based on a field value but is what I'm asking?

I added the code to the form and now I get a "message from webpage" that says:

There was an error with this field's customized event/Error: The value of the property "xxxx" is null or undefine, not a function object.

Any thoughts?

Thanks!