Vontar X4 is an android TV powered by AMlogic S905X4 processor. You can download latest Android 11 ROM for your Vontar X4 to factory / hard reset it which will erase all data stored on it.
Vontar X4 comes with 4GB DDR3 RAM and 32-128GB of eMMC storage that is expandable via micro SD card reader. Download Vontar X4 official firmware from here on a windows computer.
Then download burn card maker to create a USB drive that will help restore or factory reset the Vontar X4 android TV. Change language to English by clicking on second menu item from drop down menu. Close the app and restart it.
Choose to partition and format > Open > Make. There will be message on the screen – accept it and then start the process by clicking on the format button.
Make sure the Vontar X4 is switched off. Insert the USB drive into it. There is a flash mode button on your android TV. Press it using a thick needle or something and then connect the power cable. Update process will start automatically and it will take around 10 minutes for the process to be completed.