Realistically 3.5.
Be aware this isn't the book you think it is from the blurb.
Not knowing what to expect wasn't a bad thing though, I enjoyed this a lot to start with- good world building, and I'm a sucker for anything that really invokes familiar bits of London, as well as longhaul stories in cramped environments (but as a lot of people have said Wayfarers this ain't- possibly an unfair comparison to start with but one I definitely hoped for more of).
Found some of the bits that worked best were the simplicity of very human things (the dialogue on page 503 was so basic and matter-of-fact at an acute moment that it made me well up) but at the same time there are glaring edits that could have tautened this- repeats of sentences like "she knew he couldn't have gone far. 'He can't have gone far,' she said. 'I know,' Poppy replied."
And I agree with other people's criticisms that the oversight from the space agency and senior crew is almost non-existent, they seem to disappear from the ship for key events *and* extended periods of time so relationships and interactions happen with no one knowing, which seems unlikely to the point I started to wonder if the mission was real at all or a test in itself being 'monitored' by the senior crew, something like [spoilers] "Skinner Box" by Carole Johnstone.
Those world-breaking moments aside, this is still a fun time, but seeing other people's assertion that this is a YA title that is picked up as marketed as adult SF does make a bit more sense of what we got.