Hi @eloss,
Unfortunately, no plans to support devices with older Android version (4.1 – below) as secondary machine.
For info, please check System Requirements chapter in the user manual https://forum.spacedesk.netuser-manual/#item_445.
As an alternative, you can use spacedesk HTML5 Viewer on your old Android device.
Just open a web browser (e.g. Google Chrome) then go to http://viewer.spacedesk.net/ and connect to the IP address of your primary machine.
Keep in mind that this HTML5 VIEWER is much slower than our native apps on Android, iOS and Windows.
This is due to technical limitation of HTML5/Javascript.