hey guys, thank you for your answers
we already tried to follow the tutorial you sent sonyc2011, but i dont understant why, most of the softwares we are supposed to use just crashes.
But we found an other tutorial (type : "How to Return / Unbrick / Downgrade LG Optimus 4X HD" to find it) that worked perfectly !
So we installed the old ICS rom, we updated it to 4.1.2, then rooted it. now we are working on unlocking the bootloader. And we will THEN install the recovery
Thank you again for your help and sorry again for my bad english !