I have to disagree with you... partially, 4WD gives you better engine braking than 2WD, especially 4L and as a result is a good choice for engine braking, the catch to all engine braking though is that it gets applied very suddenly with your gear change, and as anyone who has had any experience driving in slippery conditions knows, sudden braking is a bad thing if you are already going too fast.And don't try gearing down in 4WD mode to slow yourself on any kind of slippery hill, you'll end up doing a dangerous 4-wheel slide... better to be in 2WD if you want engine braking.
so I don't recommend changing gears to slow yourself down in slippery conditions, however I DO recommend gearing down to maintain speed on the hill rather than accelerating, and doing so in 4WD will give you better control and better engine braking than 2WD, just be careful!