Is the person that rear ended me liable 4 full cost of repairs (just paint is needed)even if there was minor pre-existing damage

They are liable for the damages they caused, not preexisting damage. However, if the damages they caused can only be repaired by also repairing the preexisting scratches, you get the benefit.

The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.

They are responsible for all the damages they caused. You can ask the shop to indicate in the estimate if they can the damages caused by this and the pre-existing.