There Are Pandas, and Then There Are Pandas.
And this isn't either of them! The Pandas we're talking about here, are watches, not bears. And what got me thinking about them (again) was a link posted this morning by @cm.rook who pointed a few of us to the very attractive (and not terribly priced) Yema "Rallygraph" Panda which, in it's most traditional arrangement, looks like the one on the left, but can also be had in the version on the right: The model on the left is a true Panda, while the model on the right is called a reverse Panda. The reason for that distinction is clear--Panda bears, only come in the first arrangement. Now at this point, everyone should be thinking about the most well-know Panda, The Rolex Panda, which is actually a Daytona, and among Rolex Daytonas, the most famous of which is the Paul Newman Daytona, which was famous first, because it was Paul's, and second because it sold at auction for $17.8 million (US Dollars). The story of that auction is well-known so I'll only...
Nov 8, 2019
You can only run SLI with two identical GPUs. They do not have to be the same card manufacturers, but they have to have the same same GPU (1070 x2/1080 x2), the same VRAM amount, and same bus width. Helpful links I found for you...
http://wccftech.com/amd-freesync-nvidia-gsync-verdict/ https://forums.geforce.com/default/topic/469402/
As for whether 1070 SLI will push 60FPS on 4k, it really depends on the game. All these multi-GPU technologies depend on game developers' efforts to be fully utilized. Some games barely perform better, if not worse with SLI because the game doesn't properly support it.
I'd personally be happy with one 1080 and see how SLI support develops before considering purchase of a second card.
http://www.guru3d.com/articles_pages/zotac_geforce_gtx_1080_amp_extreme_review,21.html