Help? Looking to get more than 3 nvidia cards working on Asus Z170 Pro Gaming MB

Greetings:

I have two Asus Z170 Pro Gaming motherboards. Neither of them will run more than 3 nvidia cards. I am trying to get 4 GTX1050ti's running on one of them, but I can only get three cards to run, one on PCIex16 Slot one and Slot 3, the third card connected by a riser. If I put a card in SLot 2, it won't get detected :confused: I've confirmed this behaviour on the second system, both of them being identical in every way.

Asus tech support tells me that Slot 2 only works if the cards are in SLI. Does anyone have any experience with this configuration and what I can do to get 4 cards running?

Thanks in advance!

EDIT: The BIOS is the latest revision available.

Try updating MoBo bios and see if that helps.