I don't know exactly how zip codes work (we Canadians use Postal Codes ), but I'm assuming that there is some sort of way that you can determine which of the four regions your code is from. That being the case, you could develop 4 regular expressions (each matching one of the regions) and then you could run some code that determines which region the entered zip code matches, and then all you have to do is send them to the appropriate page.
As to the alternating page thing, you will probably already have a database of customer information right? So you can simply store (in that same database) a piece of information that tells you how many times that customer has accessed the form. Then whenever a customer goes to access the form, you see if the customer has already registered, and if he has, then get the number of times he has viewed the form, then show the form and increment the "view_times" (or whatever you call it) data. The way that you use that data to determine which form to show would be like so:
Code:
if($view_times % 2 == 0){
show_form1();
}
else{
show_form2();
}