We have a rich text field that is configured to be 4000 characters max. When I put a 3721 character field (with spaces) and it gives me an error when I try to save the record. Is there a formula to determine the "real" number of characters that can be entered?
The Rich text field is probably counting all the formatting fields such as <b> for bold etc. I assume that you have a 4000 character limit as you are feeding data to another system with limited field size of 4,000 characters. The number of characters will be depend on the amount of formatting that you do in the Rich Text field.
I suggest that you put the formula Slider suggested above on the form so the users can see how close they are to the limit.