I never drove a 4x4 in sand but i do know that i have dug into sand going up hills in the past on my 2x4 quads when i lost momentum. Gaining speed and keeping full throttle has always been the answer for me whem I have the room, But when it's not an option, making the bike lighter or using different tires may be. This may help with traction and keep you from digging in.
As for the snow, a little snow is fun, but when it gets deep it can be verry difficult to ride.
We really don't get much deep snow where i live, This is really the first season where i have a running bike and also have 6" of snow (well ice now) that stuck around. I was not impressed with the performance of my atc 185s when i went out for a ride (this was when it was still comming down). The bike would not stay in a straight line, the rear end swapped side to side, and plowing through the snow i could not get the bike out of second gear, and even when it was in second i only got it up to 7 mph or so. I found it to be more fun spinning around on the ice.
83 ATC 250r, 84 atc 250r (parting out), ATC 185s, 85 Ktm 350mxc