Apple has just released iOS 11.1 Beta 4 for registered developers. Here’s how you can download it right now.

You're suggested to back up your iDevice first, just in case. 

Step1: 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 11.1 beta4 as the following picture, finally click "Flash". (If you want to save iDevice's data, you need to select the checkbox that "Retain User's Data While Flashing")

Step3: Keep a good connection between iDevice and PC, and then just wait patiently till this process is finished. The whole flashing process needs 5~10 minutes in general, when it's finshed, then you can experience iOS 11.1 beta 4 freely.