Right. This is the typical issue in Notes. Sequential numbers per user is something easy to do. Sequential number for all docs independant of user is something inherently impossible to guarantee.
For a sequential number per user, you can easily include a Profile document for that user, and modify the form to get the number from the Profile doc, use 4999 if there is none, increment it, place it in the number field and save it back to the Profile document.
Sounds complicated, but it is not.
For a global sequential number, you need a Profile doc for the database (no user name attached to it), or a doc in a view. Let me call it the Reference document. When the form is loaded as new document, it needs to check if the Reference doc is in use or not. If it is, then the form must wait. If it is not, then the form must lock it, then do the above, then unlock it.
Implement that and, with a lot of praying every day, you might avoid having two docs with the same number.
I sincerely hope you can use a sequential number per document.
Pascal.