Giant uses a 4.5" tire on their Yukon Fat bike. That's probably best. Perhaps with a light rider like myself at 140 lbs a 4" tire would be good in dry sand too. Perhaps the same traction as a 170 lb rider with the 4.5" tire.
I tried riding my mountain bike with 2.125" tires in dry sand. Good luck with that, I got about 50' scratching and clawing my way every foot. I didn't realize those beach cruisers with 2" tires do so well in sand because they ride on the damp sand near the water. So I tried it. Sure enough, No problem with my 2.125" knobby tires.
If your riding along the shore, You might be fine with 4" or even narrower tires, Even the 2" beach cruisers use. Those Fat bikes with a 4.5" tire are probably crucial for desert dwellers where dry sand is every where. Even my comfort bike with 1.95" tires is fine for grass, soft gravel and stones (with the right gear selection) I'm likely to encounter where I live, But dry sand is a whole different story.