TinyMCE list depth limit

I was looking around but couldn't find anything interesting on the matter.

Let's say I have a requirement to allow users to create lists, but only up to 3 levels deep, like so:

1. Level 1
   1. Level 2
   2. Level 2
      1. Level 3
         1. Level 4 - not allowed, user should not be able to add 4 level
   3. Level 2
2. Level 1

The OOTB list plugins doesn't have any configuration for this and I'm not sure how to attack the problem.