Left Function

I am feeling like I have never written code before.  New front end applications will do that to you.

I am trying to apply a left function for the first 4 letters of a field and IS is telling me Left is not a function.

Here is a simplified version of what code I am using:

Left( FieldName,4) = 'XXXX'

I tried Left pad and it made no sense.

Any help would be greatly appreciated.