According to this, 4000IU/daily isn't considered excessive. My doctor did say that the 5000/IU I've been taking isn't excessive either but I'm going to mention this article to her. I have a feeling it isn't excessive enough to matter.

Taking too much vitamin D can cloud its benefits and create health risks - Harvard Health

The mice in the study discussed in the Brain article were being given 75,000IU though I'm not sure if that was daily or weekly or what. If it was daily, that seems like a very jumbo-sized dose of vitamin D for a little mouse....

I don't think they know so very much about the connection between vitamin D and MS. They think that because many people with MS have low vitamin D levels, supplementing their vitamin D just might help, and there seem to have been some benefits for some people when they've been given additional vitamin D. But they're not sure. I like to think that the added vitamin D has been helping.