Apple seeded the fourth beta of an upcoming iOS 11.4.1 update. No new features were discovered, suggesting it focuses on bug fixes and performance improvements to address issues discovered since the release of iOS 11.4. If you want to experience this new beta, you can download it using 3uTools.

As usual, you are recommended to back up your iDevice before any changes.

Step 1: Connect your iDevice to PC using the USB cable and launch 3uTools on your PC. 

Step 2: Click "Flash & JB" -> "Easy Flash", then select the firmware iOS 11.4.1beta4 as the following picture, finally click "Flash".  

• Select the checkbox that "Retain User's Data While Flashing" if you want to reserve data.

Step 3: Please do not disconnect your iDevice until the update has finished. It can take a few minutes. Your device will be updated to iOS 11.4 beta 4 and reboot once or twice during the process.