Changing columns from Nvarchar(max) to Nvarchar(4000) error

Question

I am changing columns from NVARCHAR(MAX) down to NVARCHAR(4000) the column has no need to be MAX and 4000 characters should be sufficient for the column, so for performance improvements I am making the column smaller.

I thought this would be easy like so:

alter table [MyTable]
alter column [Description] nvarchar(4000) NULL

However this gives me the error:

The size (8000) given to the parameter 'Description' exceeds the maximum allowed (4000).

I dont understand this error message.. I did not specify 8000 nor did I even exceed the maximum allowed of 4000

So whats going on here?

Edit

Some columns on tables / columns are fine with NVARCHAR(4000) , others.. are not, playing with the number 4000, the error seems to suggest the maximum I can specify on particular columns is NVARCHAR(2000)

Answer 1

Turned out to be DDL triggers as suggested by one of the comments.

These DDL triggers are provided by a thrid party for data synchronisation to the client, so I'm not 100% sure what was going on in these triggers but I'd imagine they were getting some calculation wrong and thinking the max was nvarchar(2000) rather than nvarchar(4000)