You are correct. I think I got that particular datum from one of Neil deGrasse Tyson's lectures, and doing a quick Google search I found this quote from him: "The four most common chemically active elements in the universe - hydrogen, oxygen, carbon and nitrogen - are the four most common elements of life on Earth. We are not simply in the universe. The universe is in us." Here he says "chemically active" elements, which is an important distinction, but then again he himself might've accidentally omitted it during that lecture and I didn't verify it before I wrote that piece of dialogue.
However, there's a certain thing I'd like to point out. Nitrogen comes seventh in the universe if we look at the mass fraction, but things can be a bit different if we look at the mole fraction.
If I'm reading this thing correctly, here nitrogen seems more abundant than neon and iron ->
https://www.webelements.com/periodicity/abundance_universe_a/Also, interestingly ->
https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements - if you scroll to Solar System and sort the list by atom fraction in parts per million, Nitrogen-14 is fifth again (but that's just the Solar System).
Similar thing here, but now it's sixth ->
http://www.knowledgedoor.com/2/elements_handbook/element_abundances_in_the_solar_system.html (again only for the Solar System).
In any case, thanks for the correction. I'll modify the dialogue for the next version. Even though Underrailers live underground and haven't even seen the sky or the stars in their life, she's been studying in the Oculus(!) for a while and should have good estimates; some characters are allowed to be wrong, as they often are in real life, but not in this case. Cheers!