If a web application form contain few textbox fields. Among one should accept only 4 digit according to business logic. But it's taking more than 4 digit values. Is there any potential vulnerability here?
It depends on the backend. If the application logic validates on submit, there's no problem. The developers must implement server-side checks, because bypassing client-side protection is trivial, as the example you saw.
Is there any potential vulnerability here?
Yes but it depends on how the data from that field is used and how (or if) it is validated and sanitised server side. For example, careless use could lead to overflow errors that might be exploitable.
Note that HTML5 includes many features for client-side validation of
input fields that can help.
- specific input types for numbers.
But you should always also do server side validation/sanitisation of data and especially take countermeasures against SQL-injection and similar attacks. client-side validation is mainly for acheiveing levels of responsiveness and user-friendliness that cannot easily be obtained with server-side processing.