My Doorbell 4 won’t charge from old doorbell hardwires

I use to have a Ring Doorbell 2 hardwired to my old doorbell and worked fine. The battery was always 100%. I never had to charger the battery! After 2 years I decided to upgrade to Ring Doorbell 4 and now every 2 months I need to charge the battery! Why? When I check the status is saying hardwired and shows the charging sign, but the battery keep discharging every day. I even bought a new battery. Thought the battery was faulty but still the same.! Why? What should I do? (Ireland)