I actually found out, after making some calls to a few musical repair shop, that it's actually an electronical problem for me. There are exactly 8 keys in between the 4 keys that don't work. G#, E, C, G#. And a tech told me that if it stopped working all in once, it's probably a circuit break. I just bought my Casio privia px555 10 days ago.... and I already love this piano.
I also tried to see if I can open the piano. There are so many screws underneat!! And the keyboard is the first part to be placed originally, so it's the last one when you dissemble it. I'll go in a repair shop, to be sure. But if you a bit of a geek with electronical stuff, there are tutorials on youtube that explain it
