how can i install ios 7 beta in my iphone 4??

You don't since it's only for developer. You have to wait till autumn like everyone else.

You can't. You will have to wait until iOS 7 is officially released in September time. Be patient and wait like everyone else has to.

Either this, or you need to register as a developer to download it, but you will also have to pay Apple $99 to do so, so if you are not developer, is this really worth it, when if you wait, you can download it for free when it is officially released?