If you want to use the Geniatech EyeTV Netstream 4Sat with Tvheadend and only have one cable to your dish, you can use the unicable port (Port 1) with an unicable LNB to get 4 tuners into TVheadend.
However, to enable the unicable option on the EyeTV Netstream you need the EyeTV Netstream app which you most likely cannot download from the Play Store (https://play.google.com/store/apps/details?id=com.elgato.eyetv.netstream).
I also ran into this problem but found this page on the Geniatech website https://www.geniatech.eu/de/download/android-eyetv-netstream-app-apk-version-2-1-42/ where you can download the apk file which can then be installed on your android smartphone.
Inside the EyeTV app under the settings, you can see your devices list. If you select your Netstream 4Sat device, you get the option to change the tuner / LNB settings. This is also the option you need to change if you want to use the unicable capabilities of the device.
I hope this small tutorial helps some of you to configure the device.