Lactose (milk sugar) is a non-fermentable sugar, so no, it does not serve the same purpose. It is strictly for sweetening (although the flavor isn't quite like what you associate with sweetness).
How much you prime with is up to you.
I use Northern Brewer's priming calculator
, but it doesn't list cider and I'm not sure what is correct for style on that. I always do mine still. At any rate, 4 oz of priming sugar would be safe as long as you are doing the recipe as posted and not adding any additional fermentables at bottling.