Pi 4 stopped booting after first time

I got a Pi 4 2 days ago. I attempted to install the Unofficial Ubuntu 18.04 image. This involved booting Raspbian-Lite Buster and using this to update the firmware and then booting the Ubuntu 18.04 image. I managed to boot Raspbian Lite, get wifi working and run rpi-update. The trouble is that it has not worked since. I cannot boot Ubuntu, Raspbian with Pixel, or Raspbian Lite. I have tried 2 different SD cads (4Gb and 64Gb). I have burnt the images with Win32DiskImager and Balena. I have re-formatted the SDs with Windows 10 Disk Manager and guiFormat(a tool for formatting SD cards greater than 32Gb with Fat32). Finally I have done the Firmware repair procedure multiple times and repeated all the tests.

The only other variable is the PSU. I am using my bench supply which is an ATX PC supply, more than capable of delivering 3 amps at 5V. I do not think this is an issue.

When powered on with no SD card the Pi blinks green 4 times in a continuous sequence - this indicates firmware is good I believe.

When powered with an image I see sporadic green flashes and the display shows the rainbow square, then 4 raspberries with some boot output, then blank, then repeat continuously. It also display the lightening bolt (but I have read that this does not indicate power issues at this stage of boot).

Now I am coming to the conclusion that the Pi broke or always was broken, although I do not see that I have done anything to cause that.

Any ideas ? Does this sound like a duff Pi ?

A notice here,but I’m betting on power supply

I would suggest, by way of a point of reference, to use a 16GB or 32GB (max) SD card with regular Raspbian desktop distro. This should format with no tricks needed and use Balena to ‘burn’ the image.

You will then have a solid reference SD card for testing

If this works reliably then you can’t blame the Pi - then go back to the the ‘funny stuff’ and chase.
If this doesn’t work then, yes, Pi might be suspect.

If you go here, to the Raspbian Download page, there are “Recovery” instructions to fix a possible corrupt boot loader.

The Pi 4 is the first Pi to have an actual updatable reprogrammable boot loader code in an eprom on the Pi. Once you have for sure the official boot loader flashed to the Pi, try flashing your SD card with Raspbian via Etcher. If it boots from that without issue you will know it is not a hardware issue. And can go from there.

I had complete faith in my power supply - I have used it for years on many projects. Just to eliminate doubt I got my meter and took a few readings:

Power supply: rock steady at 5.1V
Voltage at Pi 4: 4.65 V and wobbly !
Culprit: Crappy croc clip.

I wired the board to the PSU using a breadboard in the middle and it boots in seconds from Raspbian with Pixel. All issues solved.

I am kicking myself so hard. I’ve wasted 2 days looking for a software problem.

Thanks to all the people who offered help.

Been there done that,great that you got it working