You don't need to write a script. 4D basically makes or lets you create relationships on the fly as you need them. Filemaker expects the relationship to be predefined. If you had two tables, Job Table and Rates Table, where the Rates table has a list of rates for each task. Say Copywriting, Editing, etc. The Jobs Table holds data about the job, including what task was being done.
Create a relationship between the Jobs Table and the Rates Table linking them by TaskName.
For the other fields (such as Rate) that you want to auto-populate into the Jobs Table, go to the field's definition in the Job Table and set it to auto-enter by Lookup (or by calculation) the value of the rate from that relationship you just created.
As you choose a different value for the task it will auto-insert values from the matching Rates Table record.
Yep, after reading the documentation and clicking through lots n' lots of dialog boxes (Filemaker does love its dialog boxes, doesn't it?), I figured out how to do this. 4D does this kind of thing more straightforwardly if you know your code, but Filemaker does quite a bit without resorting to coding. It will take some time to get used to Filemaker's ways (and dialog boxes), but good to know that light does, indeed, exist at the end of this database tunnel. After a week's hiatus, I'll be doing some more drilling today.