Then i must have an older iPad2 and as you say an iPad4 been year's since they were bought, their batteries still working good.
I also have a over five year old Apple MacPro battery is still working good.
I also have three Samsung tablets two around five years old, their batteries still very good.
I did have a problem with a smaller Samsung tablet with a swelling battery, took a lot of calls and e-mails to Samsung until i said was going to the US Safety Gov. in regards to their battery, Samsung rebuilt the tablet installing new battery also a new charger.
One (1) year later the exact same problem happened to the tablet, after many more phone calls and e-mails to Samsung they were saying the warranty had expired, i said i am calling the US Safety Gov. then after three (3) Months they agreed to giving me a brand new tablet and it's working very well alos battery is good as of today.
IMHO i think Apple has gone with a cheaper quality battery maybe other phone Mfg. are doing the same thing who knows.
But if the batter can't last five (5) years they need to find a better quality battery, also lets face it $80US dollars for their iphone6&7 batterys and i understand it's much higher for an iPhoneX battery. Well Apple there is a lot of things going to come out of this new class action law suit that is being filed, to allow owners and third party repair without losing warranty of their phones.
Hope this will send a message to Apple and make things better for all iPhone buyers.