Bonjour Laure,
How nice that you want to
cycle the ring of Kerry! I would say that four days is totally reasonable to cycle the loop. Some people do it by bus or by car on a single day so you shouldn't have issues doing it by bike in four.
The
Discover Ireland website has an overview of the cycling route to help you measure distances. They suggest a week but it can be done in 4 if you skip a part, especially if you are a good cyclist. I also found this
alternative route covering parts of the Ring of Kerry (and Beara) along with cycling tips.
Have you looked into accommodation for the three nights you'll spend on the peninsula? I hope these itineraries help already.
A bientôt,
Antoine