Recently, I bought an used Z400 on which 48GB memory (8gb x 6) are already being installed.
It seems to work fine. Since I have not installed any OS to it yet, I have no more information now (I'm waiting for SSD that I ordered separately).
"System Information" of BIOS shows next memory related information:
"Memory Size (Total): 49152 MB DDR3/ECC/1066MHz (49152 MB)"
and
"DIMMs (MB): D1:8192 D2:8192 D3: 8192 D4: 8192 D5:8192 D6: 8192".