In earlier versions of Service Desk (lower than 14.1) the behavior for log comments was "to get a warning popup" if you paste more than the allowed 4000 chars in it. The message says "User Description contains 5123 characters, but its maximum size is 4000". With 14.1 no message comes and the text will be cut off at 4000 characters.
CA Service Desk Manager 14.1
CA Service Desk Manager 17.1
This is by design.
Using CA service Desk Manager 12.9 and IE 11 the message is displayed because the "maxlength" attribute of the textarea is ignored. This is because it's uppercase, hence the user can type in more than 4000 characters, then when 'Save' is clicked the message 'User Description contains XXXX characters, but its maximum size is 4000' would be displayed.
In 14.1 onward the 'maxlength' is enforced and the user is NOT able to type in more than 4000 characters, since they are blocked by the element.
1 - The user won't be able to type more than 4000 characters and then after save they will get the message and have to rewrite the comment.
2 - The customer doesn't know when they reach the limit.