Go Over Apple's Beta Rules & Conditions
If you decide to download the iOS 11.4 beta on your device, you'll want to go over Apple's rules and regulations before you download the software.
If you're thinking about downloading and installing the developer version of the iOS 11.4 beta you'll want to take a look at Apple's beta website for developers.
If you don't want to pay any cash for the iOS 11.4 beta, you'll need to join the Beta Software Program. Like the developer beta, Apple's Beta Software Program has some rules that you'll want to follow.
Before you download and install the public iOS 11.4 beta, read Apple's FAQ on the Beta Software Program. It goes over some of the important do's and don'ts.