oops, I already added and enabled successfully a swap partition some time ago (thanks to @egrath and his help), but I was not able to enable hibernation. I am not sure but I fear that this is due to some config of EndlessOS since my laptop hibernates easily, when I boot Q4OS from an USB stick with a swap swap partition on the stick. The swap partition survived upgrading to Endless 4 (as far as I remember, I added the swap partition under EndlessOS 3.8 end of 2020).As far as I can see others at least this and that post suggest that the authors are interested in hibernation too.
Maybe this discussion should be moved this to another topic, but I don’t know how; I think it does not fit very well to topic releases …