I’d say all of them are 4-digit codes except for one.
There’s also a Patreon post where I shared all the codes if you want to get them the “not-so-legal” way ahshash. It’s completely free, by the way, not behind a paid post.
And if you’re playing version 0.1.8, finding them should be much easier now because there’s a sound cue and a small animation/effect whenever you reach a render that contains a hidden code.
Good luck!!!