For the people landing here by Google (because it's one of the most highly ranked page), see the content of modern /etc/default/neo4j:
It is NOT used for the new systemd service.
To change these values with systemd, please usesystemctl edit neo4j.service
Most of the modern Linux distro do use systemcd
(including the above mentionned Ubuntu 18.04 - save for a few exceptions such as Gentoo's OpenRC)
You need instead to add the file limit options in /etc/systemd/system/neo4j.service.d/override.conf using the above command.