Display Contacts Custom Field in Plan Contacts Object

We have created 4 custom picklist fields in the Contacts objects. We have a requirements to display these 4 custom fields in the Plan Contacts objects under business plan.

When you add try to new record for a Plan Contact from the related information section of Business Plan, the users wanted to view the details of those custom fields from the Contact object. I have tried creating the same 4 custom fields as Contacts object  within the Plan Contact Objects and Set the default value for each of those custom fields and used the Joinfieldvalue function to pull the values from the contact object. The issue with this approach is that when the custom fields in the contact record is updated by the user,those updated values do not get