ext4-modules-4.7.0-1-arm64-di software package provides ext2/ext3/ext4 filesystem support, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install ext4-modules-4.7.0-1-arm64-di
ext4-modules-4.7.0-1-arm64-di is installed in your system.
Make ensure the ext4-modules-4.7.0-1-arm64-di package were installed using the commands given below,
$ sudo dpkg-query -l | grep ext4-modules-4.7.0-1-arm64-di *
You will get with ext4-modules-4.7.0-1-arm64-di package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | debian-installer udeb packages |
| Package | ext4-modules-4.7.0-1-arm64-di |
| Version | 4.7.8-1 |
| Description | ext2/ext3/ext4 filesystem support |
| Sub Section | Debian-installer |