My PS3 version is SLIM CECH 2504A. I bought it in December 2010 so there should be no problem in 3.55. Besides, before bricked I was able to downgrade from 4.00 -> 3.55 and when I run PS3 in Service Mode I checked the versions and it was 3.55. Then, unfortunately, I something messed up with files on the USB stick and then PS3 got bricked.
As for your advice, all worked until the sentence "If all went well you will boot into 4.0". In fact, I can start the console (with my BIOS) and 6th switch UP (rest is down) but still no HD/USB activities... When I upload e3_downgrade.bin to SD card, hit reset and start - then I get the same error 11000100.
I even tried to patch my BIOS according to this guide (http://www.ps3hax.net/showthread.php?t=30815
) and upload it but after that the console will not start at all ...