How can I mount an ext4 file system on OS X?
The answer depends on you willingness to invest in commercial software:
If you don’t mind spending some money on a commercial product, Paragon’s extFS for Mac will give you read and write access to ext2 / ext3 / ext4 file systems. The current version supports all versions of OS X / macOS from 10.10 upwards.
If you are looking for a free solution, you can setup a Linux virtual machine, mount your volume(s) there and share it / them via Samba or (S)FTP. This post has some details on how to achieve this using VirtualBox, a free virtual machine application. Note this is not exactly a lightweight solution, even if using a prebuilt VirtualBox VM will spare you installing and configuring a Linux distro from scratch.