Merge Event state_id meanings

I’m trying to check if a merge request has been merged using the merge requests system hook. The changes object returned has state_id, but I can’t find any information about what the different numbers mean anywhere in the documentation or anywhere online.

This is what I’m seeing:

changes: {
 updated_at: {
  previous: "2021-01-12 11:30:22 UTC"
  current: "2021-01-12 11:30:22 UTC"
 state_id: {
  previous: 4
  current: 3

I’m using self-managed Starter edition GitLab version 13.5.1-ee