Acer Iconia Tab W4 820 is one of the cheapest Windows 10 tablets out there. It came out with Windows 8.1 back then but Acer did provide upgrade to Windows 10.
This was a great gesture from Acer since its predecessor, Acer W3 could not run Windows 10 due to lack of graphic card drivers. We blame Intel for that though.
Anyway, if you want to clean installation of Windows 10 / 8.1 on Acer W4 820 tablet, you will have to follow these steps.
1. Make a bootable version of Windows. You can use Microsoft ISO to USB tool for that.
2. Fully charge your Acer tablet. This is important as unlike Acer W3, there is no dedicated charging port. You will also need USB hub as you need to connect USB mouse and keyboard. Touchscreen does not work while setting up windows.
Plug in bootable USB drive that you created in step 1.
3. Boot into BIOS by pressing volume down and power button for few seconds. Then, set a SuperVisor password. Then, go into boot settings and disable Secure Boot. Move USB drive all the way to top.
Do the windows installation and once it completes, change boot preference to internal storage again.
Install drivers by downloading them from official Acer’s website. Windows 8.1 32-bit drivers are compatible with Windows 10.