@Koenraad, the vertical justification settings were indeed what you want, but you were caught in a trap that is one of Dorico’s current weaknesses: justification is for staves and notes, but it doesn’t currently take lyrics into account.
The way around this is to set the proper justification settings (in this case 70% is enough, and then also change the music frame bottom margin to something large enough to accommodate your lyrics. (Hopefully future versions of the software will take this into account automatically.) In this case 0.4" works for me.
When you do this at first, it thinks that the frame is too full, so the 4th stave pops onto the next page. To fix this you can use frame breaks, or, to keep it all automatic, lower the default distance between staves so Dorico doesn’t think the frame is too full. In this case, lowering the inter-system gap to 7 takes care of it. Alternatively, you can use casting off options to force 4 staves per page as well. This basically overrides some of vertical spacing settings.
It’s learning how these all work together that takes a bit getting used to, but once you do you’ll see how powerful they are.