Personally I would recommend you to use the official importer instead (It produces a much better quality terrain than this plugin).
With the program itself, make sure you’re running the .jar application, not the .java file, and I’ve heard there were some issues with running the program on non-windows machines. Also make sure you heightmap and colourmap are the same resolution and are both a square.
So much better than the one that Roblox made
Can the official importer import a 4096x4096 heightmap?
This is really, really nice. I look forward to using this next time I work on a project.
How good is this compared to the default heightmap importer?
As he stated in the post, this might slightly be worse, still worth it as it looks, why did you bump this.
I want to know if this is different from the default importer.
This is not being worked on or supported any more, I would personally recommend you to just use the inbuilt terrain importer, as it does a much better job compared to this plugin. I never managed to figure out how to fix some of the weird artefacts created with terrain using a large scale - the inbuilt importer does not have this issue.
However you are always free to look through the code and make any modifications you wish.