This is fixed in the next release. You will be able to create an area with umlauts in the name after upgrading to 4.0.2. In other cases (not just area titles), such characters were being double-encoded if certain actions were performed. For example, undoing a page to a previous edit would cause the non-ASCII characters in its title to become double-encoded. I am surprised that this gives a not-found message for the area -- I would expect it to open with the title messed up.
I would advise you to create the area without the accent marks for now. Then rename it after 4.0.2 comes out.
It should be fine to create regular pages with umlauts in titles right now as long as you don't undo any edits to those pages.
Due to this issue resulting in mixed encodings, it's not possible for the 4.0.2 upgrade script to detect what encoding is intentional or unintentional. Thus it is not possible for the upgrade script to fix affected entries automatically without the potentiality of creating security vulnerabilities in random titles. After the upgrade, you will simply have to rename any content manually that looks like it was affected by this bug.