ok, thanks James,
I have tried again with a Zenpack that is officially supported on Zenoss 4.2.4, namely ZenPacks.community.InterfaceGraphs. I could install it using
git clone https://github.com/zenoss/ZenPacks.community.InterfaceGraphs.git
zenpack --install ZenPacks.community.InterfaceGraphs
and it even works, congrats to Simon Helson for providing this ZenPack.
My problems are probably linked to the fact that very few developers have provided 4.x versions of their ZenPacks.