With limited info and with assumption that Other devices like windows Laptops and smartphone work ok, would like to suggest some steps to narrow down the issue:
1) what is the status of client on ZD when they fill their credentials but nothing happens? If they are authorized then close the window and try a new one to see if it works or not OR when "nothing happens" on the browser just remove the current URL and go to any website.
2) if above step fail then try a different browser on Iphone 4.
Best of luck.
PS: I am not a Iphone user