I adopted a 4 year old Scottie that was part of a huge puppymill rescue in my area. How, if ever, can this dog be pottytrained, as he has spent the last 4 years "going" in a cage?

it is very possible that he will never be, that is part of the problem with mill dogs.

you are going to have to start at square one - every hour take him out on leash to a particular spot. use a one word command consistantly.

get a bale of straw & take one or two flakes & fluff them up & put them on the spot you want him to use. lots of loving praise when he does it right but say nothing when he doesn't - just clean it up

be sure to use a good enzyme cleaner, or vinegar & water to neutralize the spots so he is not attracted to them.

if you are feeding a grocery store food he is pooping more than he would on a quality food