Four blinks of the green led indicates the system can not find (maybe corrupted) some starting files (.elf)
Few days ago, after an upgrade I got seven blinks when rebooting. This indicates the system can not find the kernel.
i had to burn again the stellarmate image. There are methods to recover the system without losing your files but I found them a bit complicated. In my case, I mounted the sd card on another raspberry and copied the important files before burning the image.

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