If you can go up to a 4.0 rather than a 3.8, I recommend looking into Schwalbe Jumbo Jims. Lightest and best rolling fat tire I could find. Been running them for several years on my wife's Salsa Mukluk & she loves 'em. I'm running 2.6 x 27.5" Schwalbe Rocket Ron's on my Mukluk for a slightly more nimble ride. I've not found better rolling tires in those sizes than Schwalbes. Don't take my word for it -
check out the real data!
Howard