Input 4 first digit from column A to column B

Hello everybody.
This is simple, i had goggle but havent find result that suit me.
What I want is something like this. The column A is A_number, and column B is B_number.
So I want this condition : for example when user type input 123456789 into column A, the column B automatically filled with 1234
How to do this automatically?

If i understood properly what you meant, here is the solution with native js:

http://jsfiddle.net/y3whbn4o/1/

Is it possible to do that using php only(without js)?

PHP Code:

string substr ( string $string , int $start [, int $length ] ) 


try this...

So you want to access first 4 digits, regardless of what the number is?

Could probably get away with using string functions and literally get first 4 characters out of the string.

PHP Code:

$a = 123486868686868;
$aString = strval($a);
$b = substr($aString, 0, 4); 


All POST data comes in as string anyway, and PHP is quite good at converting types in and out as it is, but I added extra step in there just to point out string conversion.