Hi, I woke up this morning to the very same issue after my iPhone pushed the ios 14.4 update through last night. It was stuck on the apple logo and then suddenly the screen would flash off, over and over. I put the phone in recovery mode and plugged it into my computer to try and update or restore the phone and got error 4013 every time. I tried every solution on this forum and others, but nothing worked. However, a guy at the UBreakIFix store figured it out. He said he did something where he unplugged the Face ID cable and let it restart and then plugged it back in. He only thought of doing that after I told him that my Face ID was broken and I had been typing in the passcode for the last couple of months. Once he fixed it he told me I cannot let my phone die or update again otherwise the boot loop will return. We also made sure my front camera and audio still works. It seems like a pretty similar solution to Xannidax. Hope you can take this to a 3rd party and get your phone fixed!