Ext4 isn't used on Android, and it isn't immune to fragmentation.

The way ext4 reduces fragmentation is with some basic heuristics: mainly, it spreads files across the full disk instead of finding the next free spot. So they have room to grow without fragmenting. When the space gets low, it fragments just as badly as older file systems unfortunately.

My phone at least does use ext4 according to the output of `mount` in termux.