Create a 4-digit security code input that allows you to enter a two-factor authorization code. It should resemble the screenshot below:

Implement a form submission handler that calls a submitCode(code) function with the 4 digits as a concatenated string. Implement a submitCode function that validates the code given against a hardcoded 4-digit string.

The inputs should be as usable as possible, specifically:

1. Each field should allow only one digit between 0-9. Any other input should be rejected
2. Entering a number in a field should advance the cursor to the next field, except in the case of the last field
3. Pressing backspace at the beginning of a field (whether that field is populated or not) should focus the previous field and delete the input inside
4. Very basic styling guidance:
   1. Inputs should be positioned next to one another
   2. Inputs should be roughly rectangular as pictured
   3. Submit button should be positioned below

Note: Pasting input into the field(s) is a complicated problem, addressed in one of the additional challenges. For the main problem, assume that copying and pasting a number into the field(s) results in undefined behavior.