The CreditCardNumberLastFourDigits field maps from the Donations table in Luminate Online to the corresponding table in LCRM. However, this field doesn't exist in the Recurring Gifts table in Luminate Online, which is why that data doesn't show up on the Recurring Gift record in LCRM. This means you will only be able to see the last 4 digits of the credit card directly on the donation record, not the recurring gift or pledge record that holds the donation.
To ensure that you have the fields visible on the donation record in LCRM, click Edit Layout on any donation record and add the "Card Number" field to the layout.
If you want to add the field from the Donation object to the Recurring Gift object, you will need to add a lookup field.
To do so: Go to the field list for your custom object and create a new field. When you get to the formula editor, click Insert Field. From there, find your lookup to Opportunities. It will have a > sign to the right of the field name. When you click that, a new pane will open to the right will all your Opportunity fields there. Just select one, insert it, and you're set.