I think it's worth noting how light fatbike frames are nowadays — and I'm not talking about alloy, even the alu frame on my cargo fatbike (Blackborow) is extremely light. The tires are reasonably heavy though, and this contributes to the amount of effort required to spin them up. So instead of worrying about frame weight, I'd worry more about the wheels — this is where you want carbon rims and a tubeless setup, assuming you can afford it, of course. Also, and purists will hate me for this, consider using 3.8" tires instead of 4.8" ones... yeah yeah I know the choice of tires is a lot more limited, but this tradeoff has very concrete advantages: apart from the slightly decreased weight, you can also run a front Fox 34 tire on a 65mm rim. The upside of this is you get a lighter, much better-performing Fox 34 fork instead of the canonical Bluto/Mastodon. It's lighter, the wheel is lighter. It's up to you whether you want a 4.8" or 3.8" rear but I find that 3.8" is 'big enough' for purposes of swallowing rocks. And yeah, sure, 4.8/5.0 tires will have more traction.
On a side note, if you have an electric fatbike, weight doesn't matter so you can go totally all-out on the wheels. Just make sure you get 203mm front and rear rotors (composite, preferably) and dual-piston calipers.