PHP Form with Last 4 Digit Validation

I need a PHP form (a single text/number field only) that will pass the user to a specific address, but only if the last 4 digits of the code they enter into the form matches 3471 (example number). I understand basic forms, but not to this extent [hint: I not a coder, but can get a round a bit].

For example ... when the user enters "12345678903471" into the form they will be taken to http://sample.web ... else the page simply refreshes with a statement "Incorrect code"

THX all!