I have had several batteries that lasted a more like 7 years in standard daily driving. I had one in a stored vehicle that lasted 11 years with occasional starting and float charging. I've also had some fail in less than 2 years and replaced under its partial warranty (pro-rata). The worst was a Penn-Deka battery, and since Johnson Controls spun off their motive battery division I don't know who makes a good one any more.
Four years is not unreasonable, but most batteries I've had begin to fail in cold weather, not hot. I usually replace them just going into winter, like November-December since even a weak one can limp through summer. I suspect your area probably doesn't vary in temperature that much so the phenomenon may not be parallel to that.