For the first time users can jailbreak iOS 11.4 beta 1, 2 and 3, thankfully iPhone 8/8+ and iPhone X users can downgrade to iOS 11.4 beta 3*.
*It is important to note that downgrading from your current version of iOS is only possible whilst Apple is still signing the older version of iOS firmware, once that signing window is closed then downgrading will not be possible*.
Before any attempts of downgrading are made make sure you backup your data like Photos and stuff to iCloud or locally to your PC before proceeding, note that full Backup option available in iTunes and iCloud won’t work if you try to restore a backup made from a newer firmware onto an older version.
Download the version of iOS firmware file from here.
Start iTunes, head to the device’s.
Now hold down the Option / Alt key on your Mac’s keyboard, or Shift key if you are on Windows, and click the Restore iPhone button.
You will now be prompted to browse and select the IOS ipw file you want to download, select beta 3 of iOS 11.4 that you downloaded earlier.
Select and open the file, which should begin the downgrading / restoring to iOS process.