I have to ask why there are two tables - based on what you’re describing, the Coda way of doing it would be to create a view of the 20-column table, with all but 4 columns hidden. That way both tables are always in sync, data can be input/updated in either one, etc.
But perhaps you have a good reason for the two tables thing. In that case, you could have an automation with a Row Changed trigger, and then keep track of whether a row is new or not using this technique (I know… “new row” should be its own automation trigger!)