[Sony Xperia 1 IV] sebaro:
Thank you again for your outstanding help.

I tried the procedure and hold volume down while connecting it to the pc.

My PC made that normal USB connect sound, the LED first went red then green and stays green.

So it seems that it goes directly to the flashmode again?

Edit: So I tried force shutting it down and it vibrates three times. After that I hold the volume down button and connect it to the PC. But then the LED stays green.

Edit2: Okay managed to boot the device up and got then into bootloader mode. I think I have to reinstall the ADB Drivers on my PC since "fastboot devices" does not show anything even when in bootmode with blue led.

Edit3: Sorry for the trouble. I reinstalled the Android ADB drivers and after that the device was shown in fastboot and I managed to flashed the patched boot image. Now everything is working again.

The last edit:

After running through the process of updating my Xperia 1 IV I thought about writing a short tutorial for all struggeling with the procedure. I combinded the steps from @radiosamurai, @Doom Slayer and the tutorial form Huenet.

Prerequisites

Google USB Driver (for ADB Connection): https://developer.android.com/studio/run/win-usb

Installation of Google USB Driver: https://developer.android.com/studio/run/oem-usb#InstallingDriver

Android Platform Tools: https://developer.android.com/tools/releases/platform-tools

XperiFirm: https://xdaforums.com/t/tool-xperifirm-xperia-firmware-downloader-v5-6-5.2834142/

Newflasher: https://xdaforums.com/t/tool-newflasher-xperia-command-line-flasher.3619426/

Unsin: https://xdaforums.com/t/tool-unsin-sin-v3-v4-v5-unpacker-v1-13.3128106/

Extract all Tools and Drivers

Update Process

Follow the tutorial from huenet: https://blog.huenet.net/updating-a-rooted-sony-xperia-1-iii/

Before moving everthing as stated in section 4, you should make a backup of the whole firmware folder to another (sub)folder or drive. After that unsin the .sin files to extract to boot.img file of the new firmware. You should only do this with the "work in progress" firmware folder.

Move the UnSin tool in the same directory as the headfolder of the new Firmware and open a cmd window in the same directory.

(We use Unsin 1.13)

With

unsin.exe -dr

the process of extracting the .sin files should start.

After that there should be a "boot_X-FLASH-ALL-*.img" in the firmware folder.

This file should be transfered to you phone and patched via magisk.

Todo so open the magisk app -> Magsik Install -> select a file and patch -> choose the copied "boot_X-FLASH-ALL-*.img" file.

After the patching is done you have now another file called "magisk_patched-*.img".

This file should be transferred back to your PC. We need it later on.

Since you unpacked all .sin files to get the boot.img, you should now delete this firmware folder and work with the backuped one before, to have a fresh base with no bloat.

Now after you patched the boot image of the new firmware, you can proceed with the remaining steps of the tutorial from huenet starting at section 4 until you reach section 10.

Here you choose the fastboot option to boot the device into fastboot after flashing.

Proceed with the remaining steps and after on the device should be booted into fastboot (blue LED should be steady on).

Now you open a cmd window in the folder where your ADB Android Platform tools are.

With

fastboot flash boot magisk_patched-*.img

the rooted boot image should be flashed to the device.

After doing so type

fastboot reboot

to reboot into the normal Android system.

Congratulations! You have successfully updated you Xperia 1 IV and preserved to root rights

If you cannot find a device at fastboot command make sure the bootloader driver is installed.