The "mah" of the battery determines how long the battery will last, and that would decrease in time over years.
The "A"of the charger determines the speed of how quickly it charges.
Most phones with 4000mah batteries would be supplied with a 2A charger as a minimum.
Most mid range and up phones have a 1.5A charger nowdays.
You can still use older 0.8A chargers,it just takes very long to charge, that amperage is still higher than the trickle charge the phones default to accept when the software notices the battery is fully charged.
These are basic laws of electricity and can't be changed,
