I think four years is a pretty good and acceptable timespan for a cheap chinese noname motorcycle battery. I would not expect them to live so much longer than that.
As a replacement I would seriously think about a LiFePo4 from Shido or similar quality products. They are absolutely maintanance free. No battery charger/tender needed. Install it, ride it, smile and be happy. My Shido is doing its job flawlessly every time for over 2 years now without taking any care. 3 years warranty by the way.