BambiBoomZ, please reread my comments..... if the Z4 G4 does use a multi rail power supply instead of a single rail supply then overloading the GPU rail is a real possibility as the 1000 watt supply only has two six pin aux GPU connections and i don't know if each gpu connection is on a seperate rail or what the gpu rail(s) are specd at because as i said i don't own this system and have no experience with it
http://www.dynapowerusa.com/difference-between-single-rail-and-multiple-rail-power-supplies/
HP does spec the 290 watt Quadro RTX 8000 card as a option, but may include a gpu power adapter to take power from another rail besides the aux GPU one or they may just include the HP 6 to 8 pin adapter that is a overengineered/rated adapter that uses thicker wiring and better quality molex connectors compared to most of the generic 6 to 8 pin adapters
i do know that adding a high power card also requires the optional HP front cooling fan kit
it's also possible the "OP" is also using poorly made 6 to 8 pin gpu adapters