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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Can you provide us more information on that first four digits of a number sequence so we can understand how it shall be computed?
Anyhow, you can try using the Form Calculation Widget and set your formula there, together with the corresponding field that you're using. You can refer to this guide to learn more about Form Calculation Widgets: https://www.jotform.com/help/259-How-to-Perform-Calculation-in-the-Form
We'll be looking forward to your response, should you need further assistance.