the z820 supports SLI and can support up to 3 video cards however HP does not offically support 3 way SLI
also the M4000 is not offically supported on the z820, (it's listed as supported on the z840) but i see no reasion it should not work on the z820 but i have no experence with this card so i can't say for sure
next, does your M4000 have DUAL SLI CONNECTORS? if not then there is no way to do 3way SLI
and finally, even if the quadro card has SLI connectors they might not be enabled, some quadro models share the same board with other models but on the lowend model SLI may be disabled
from what i can see on the PNY web site ( the card maker for NA) it does support dual SLI but as i said the sec SLI connector may not be enabled on this model
i would call PNY tech support and see if they have a answer