Absorption status effect gives 2 extra yellow hearts per level. 

Amplifier 0 gives the default 2 yellow hearts

Amplifier 1 gives 4 yellow hearts as level 2

Amplifier 2 gives 6 yellow hearts as level 3

Amplifier 3 gives 8 yellow hearts as level 4.

Amplifier 3 or level 4 with 8 extra hearts is what enchanted golden apples gives.

But status effects have maximum levels up to 255, achievable through commands or addons. Each amplifier adds 2 yellow hearts.

Currently, the yellow hearts acquired from the absorption can only go up to 8 hearts despite setting the amplifiers beyond 3 or having absorption effects more than level 4.