I was facing same boot loop issue on my iPhone XS Max when I was installing iOS 14 beta. But finally, I'm able to resolve the issue. Follow below steps, I hope it will work out for you as well.
First of all, connect your iPhone with your Mac or PC through USB or USB-C. However, my Mac wasn't detecting my iPhone for me. But as soon as, I put my iPhone into DFU mode. It started showing up. Follow below steps to put your iPhone into DFU mode(For iPhone X and above only).
Quick-press the Volume Up button.
Quick-press the Volume Down button.
Press and hold the Power button until the screen goes black.
Release the Power button once the screen blacks out, then
Quickly press and hold the Power and Volume Down buttons together for 5 seconds.
After 5 seconds, release the Power button without letting go of the Volume Down button.
Continue holding the Volume Down button for at least 5 seconds.
After following above steps, my Mac was able to detect iPhone and I reset my iPhone with latest iOS version(13.5.1). Luckily, I had backup so I was able to get all my data back. After that I downloaded iOS 14 beta 4 again and updated iPhone to 14 beta 4.
Note: If you get error when you are trying to connect your iPhone or reset to latest version then please try same process again and again. I will work out.