How can I create an auto-incrementing ID field for a web app?

I have a web app for a project database. Customers can login and add a new record to the web app. We need it to provide each new project with a unique and sequential number. The BC ID number for each web app item is unique but of course not sequential (and too long). So for example the client would like all project IDs to be 4 digits starting from 1000. Any ideas?

Thanks in advance!