Super cool Sagpeter ! 
First of all, congratulation for your new boat, welcome aboard ! Looking forward for some pictures of your boat, navigations and if applicable your works as well :)
Unfortunately, these categories didn’t exist in the 70’s. CE Certification appeared in the late 90’s. So on our papers there is no such mention, and it’s alright for us. For once, the French administration don’t feel the need to handle impossible situations… :).
There used to be a french regulation on boats that disapeared when CE Certification was introduced : The « Catégories de navigation ».
As far as I know, the Edel 4 was in the third category (« Catégorie 3″, 3ème catégorie », this should be mentioned on your boat identification tag, usually close to the motor).
There are no exact match between the two reglementations : they don’t focus on the same aspects.
« Catégorie 3″ meant no further than 60 nautical miles from the shore.
The CE catégories are based on the wind speed and wave height the boat could face.
Nevertheless, French »Catégorie 3 » should broadly compare to the CE « B » Category. But again, there is no direct possible comparison. And personaly I use the boat mostly as a C category, over 25~30 knts of wind I prefer to stay to the bar. These are very well built boats but quite old too :)
If you want to go to the 60 miles limit, you’ll need all the required security equipment aboard (including a liferaft that is often more expensive than the boat herself), as required by the ColReg (RIPAM in french).
The google translate of an article from a french magazine on this subject :