Using 64-bit arm64 image
What should we do:
1. Install updated bootloader into RPi, see this guide 51 for complete instruction.
2. Download official installation image to laptop using link https://ubuntu-mate.org/download/arm64/focal/ 14 and write it to microSD card using USB- or internal cardreader.
3. Boot Raspberry Pi 4 using microSD card as usual: choose locale, create user, reboot, login to system, then install all updates and do cleanup by sudo apt update && sudo apt dist-upgrade -y && sudo apt autopurge -y && sudo apt clean -y, reboot to ensure that system on microSD is fully functional, poweroff the Raspberry Pi 4.
4. Connect microSD card to laptop, mount system-boot on laptop and then replace the contents of config.txt file with the following lines:
[pi4]
max_framebuffers=2
[all]
arm_64bit=1
# kernel=uboot_rpi_4.bin
kernel=vmlinuz
cmdline=cmdline.txt
initramfs initrd.img followkernel
# device_tree_address=0x03000000
# enable_uart=0
# Enable the audio output, I2C and SPI interfaces on the GPIO header
dtparam=audio=on
dtparam=i2c_arm=on
dtparam=spi=on
# Enable the FKMS ("Fake" KMS) graphics overlay, enable the camera firmware
# and allocate 128Mb to the GPU memory
dtoverlay=vc4-fkms-v3d
gpu_mem=128
start_x=1
# Comment out the following line if the edges of the desktop appear outside
# the edges of your display
disable_overscan=1
# If you have issues with audio, you may try uncommenting the following line
# which forces the HDMI output into HDMI mode instead of DVI (which doesn't
# support audio output)
# If you have a CM4, uncomment the following line to enable the USB2 outputs
# on the IO board (assuming your CM4 is plugged into such a board)
#dtoverlay=dwc2,dr_mode=host
5. Insert microSD card to USB-cardreader, connect USB-cardreader to Raspberry Pi 4, allow it to boot, then power it off.
6. Copy the whole microSD card to USB-flash or USB SSD, optionally grow the size of writable partition to fit free space of USB media.
7. Connect USB media to Raspberry Pi 4, boot it and enjoy!