Wow!! There was at least one "never registered" 2CV but I had never heard of a R4.
There is no answer on "how much is it worth" except for putting it on ebay and see...
And there will always be the question, is it better to use it normally and anjoy a (really) new R4 as you would 20 years ago, or keep it in a garage, so that it keeps its freshness forever?