How to mount EXT4 disk on 10.15?
Mac does not support Ext4 file system. If you plug in a hard drive, Mac won't recognize it. Fortunately, there are several ways to handle this situation.
1. Temporary options: Use VM
Just install a version of Ubuntu, or whatever your Linux distribution of choice is, in a virtual machine host like VirtualBox, then mount the drive as you would any other and read away.
2. Add Ext4 support for macOS
If you regularly use Ext4 formatted drives and / or need to copy multiple files from there to the macOS drive, you need a better option.
You need to install some software, namely osxfuse and ext4fuse. The easiest way to install them is to use Homebrew.
After Homebrew is installed (or if it is already), run the following commands:
brew install --cask osxfuse
brew install ext4fuse 

After installing the Ext4 support software, you now need to determine the hard drive you want to mount. To do this, run the following command:
diskutil list 

Save the partition ID (will look like /dev/disk3s1). Then, run the following command to mount the hard drive:
sudo ext4fuse /dev/disk3s1 ~/tmp/ext4_support_PARTITION -o allow_other

ext4_support above can be any name you choose. Now, navigate to the '/ tmp /' directory in the Finder and you will see the contents of the partition listed. If your drive has multiple partitions, you can mount them using the same steps as above. Just make sure to use different directory names to mount them.
Warning
Although these tools can help you read Ext4 formatted hard drives, they are not stable. As long as you are mounting read-only drives, as what is being done in this solution, you will not have many risks. If you try to use these tools to write to an Ext4 drive, you may lose data.
If you need to move files back and forth on a shared drive with Linux, this method is not recommended. Instead, use another file system like ExFAT or try the commercial option listed below
3. Use Paid Software
Software such as Paragon offers a free trial version, but to be safe, you should back up your hard drive first, in case there is a problem. If you want to buy software, it is available for 40$
Note: I haven't used this tool and can't say anything about what they have promised.
Although reading Ext4 format on macOS is no longer an impossible task, it is frustrating when Apple does not support this format.
Try it on your own risk but I strongly recommend the first solution which is easy and convenient.