I looked at the code in the asset that xyz linked.
Use the file Big.gd for Godot 3, and use the file Big.gd4 for Godot 4. (I figured that out by noting that Big.gd uses Reference, which is in Godot 3, and Big.gd4 replaces it with RefCounted, which is in Godot 4.) The last update was May 26, 2023, so it's probably up to date.
The code is just math and string processing, with no nodes, etc., so if there are any other conversion issues, fixing them should be easy.