tels you what four flashes means, basically one of the files in the boot partition (the partition in FAT32 readable by the boot code, and windows) is corrupt.
These are the other patterns that the ACT LED might show during a failed boot, together with their meanings:
3 flashes: start.elf not found
4 flashes: start.elf not launch-able (corrupt) See below:
7 flashes: kernel.img not found
8 flashes: SDRAM not recognized. You need newer bootcode.bin/start.elf firmware, or your SDRAM is damaged
so the file start.elf is damaged.
Best solution, use the etcher program to write a raspbian image on it, as described here: https://www.raspberrypi.org/documentati ... /README.md
download the Raspbian image from here: