Where I can find the drawing of the PCB (all 4-layer)?
It is unlikely you will get the PCB layout from the designers, although you should be able to considering it is supposed to be an ‘open source’ project.
The bulk of the IP is in the schematic and software/firmware, and all these are available. With the schematic in hand, it would be possible (although tedious) to reverse engineer the layout.
It is not what you are after, but it is an indication. The current layout is reasonably similar…