So with the launch of the Nvidia RTX A6000 and Nvidia A40 GPUs, I noticed their product pages mentioned an SLI configuration that can still go beyond 2-way SLI.
https://www.nvidia.com/en-us/design-visualization/quadro/rtx-a6000/
https://www.nvidia.com/en-us/data-center/a40/
Under Professional Features and VR, you will find the following:
"Four-way explicit VR SLI enables peak performance, assigning two NVLink-connected GPUs to each eye."
 
Considering these cards are using the same GA102 as found on the GeForce RTX 3090 (different drivers, more cores active), I'm wondering if this capability exists for the 3090. We only heard about 2-way SLI since that is all NVLink supports, but I don't think VR SLI needs a bridge between the card or cards that drive each eye according to this Nvidia paper (see page 11):
https://developer.nvidia.com/sites/default/files/akamai/gameworks/vr/GameWorks_VR_2015_Final_handouts.pdf
"And of course we need to be able to transfer data between GPUs. For instance, after we’ve rendering both halves of the stereo pair, we have to get them back onto a single GPU to output to the display. So we have an API call that copies a texture or a buffer between two specified GPUs, using the PCI Express bus."
 
If anyone knows if the 3090 can support this configuration too (or at least VR SLI with 2 GPUs without a bridge), that would be awesome to know. That said, I don't think there is any current practical application to owning more than 2 3090s. From my reading, of all the games that support VR SLI (there are only a few), none support explicit DX12 multi-GPU. And of all the games that support GPU PhysX, none support either explicit DX12 multi-GPU or VR SLI (except Nvidia's VR Funhouse which is VR SLI + GPU PhysX). But some game developer may come along and change things in order to impress that <0.01%.

Of course (literally) any GPU can be used in a quad explicit mGPU configuration in DX12 - if the game developers coded for it, which almost none do. Four 3090s would probably not work, though - as there have been reports of the NVlink bridge being required to get things to work properly. But if the game allowed you to assign specific GPUs to each eye, you could break up the GPU pairs, each with their own NVlink bridge.

It would be an interesting tech demo, but kinda pointless because there would only ever be that single VR game to do it (at least judging by the near total lack of DX12 mGPU game developer enablement thus far).

I found an interesting article that also supports the theory that four-way explicit VR SLI support for the GeForce RTX 3090 is impossible. Such a setup would require dual NVLink connections, as mentioned on the nVidia RTX A6000 and nVidia A40 product pages. But according to Pudget Systems, support for dual NVLink on GeForce was never intended, although it was last supported unintentionally with 430.86 which was released back on May 27, 2019, too old to support the new GeForce RTX 3090.
https://www.pugetsystems.com/labs/articles/Unsupported-How-to-Make-Dual-NVLink-Work-on-Windows-10-1688/
 
So if you want 4 current GPUs for graphics, it looks like you are limited to using the Quadro lineup. Ampere GeForce caps out at 2 (+1 for PhysX).
While Pudget Systems failed to get dual NVLink working on their Quadros using current drivers at the time, considering nVidia's product pages for the new Ampere Quadros, I'm guessing those problems have been fixed. I sent a query out to nVidia though just to make sure and will update when I get a response.

mchang1984
Seems unlikely there will be one person with four 3090's to give you some kind of feedback on this since most of us can't even buy a single 3090 due to out of stock situation.. I would just email NVidia customer support for an answer.

Also, when you get 3 or more 3090s, you can't fit them into one system without PCIEx16 extension cables unless you watercool all of them to make them narrow enough to fit.
 

 

sparetimepc
Well darn, that's alot of gpu power and cables. What are you using this pc for?

3D rendering. 
 
I also used it to break the previous 3D rendering benchmark world record I set last year.  I previously set the record with a pair of water cooled 2080 Supers and a 3090.  The system with the 2080 Supers and 3090 beat 4 quadro RTX cards in an Nvidia Iray benchmark.