This is wholly a guess, but here are my thoughts. It will/should work, but it might be a long way around it.

This is an outline of what you could do.

Set a variable x=0. When you depress any number, the count increases by one. Check the number of keypresses after you increase it. Hence, when you press the first number, the number increases to 1, and then checks to see if the count is 4. It also sets a value of that number to a variable. When you press the second number, it increases to 2, and sets that number to a variable. Same with the 3rd and 4th, but then when count equals 4 it then checks to see on another variable.

var NumTotal = number of keypresses
var NumCode1 = first key pressed
var NumCode2 = second key pressed
var NumCode3 = third key pressed
var NumCode4 = fourth key pressed

var Code1=2
var Code2=7
var Code3=3
var Code4=5

if NumCode1 = Code1 then
if NumCode2= Code2 then
if NumCode3=Code3 then
if NumCode4=Code4 then
open the webpage
else
call them a loser and send them home to mama


I know, it's rough, but it's only meant as an idea, not full working code.

If someone sees an error, please fix.

I hope this helps