RE: Hemi 4/8 cylinder thought.
I don't know the definite answer, but, the spark plugs are supposed to be replaced every 30,000 miles on the Hemi. So, I don't imagine spark plug wear is a major concern. But even more importantly, from what I've read, the cylinder deactivation is very minimal on a Ram. The only time I can imagine running on 4 cylinders is driving on the freeway, downhill, with the wind behind you. If you are driving fast, uphill, or into a crosswind, the truck is probably running on 8.
I think if you were to see 4 cylinders new and 4 trashed, you'd have to do 99.99999999999999999999999999% of your driving at highway speeds, coasting downhills. Of course, this isn't practical. Personally, I think the cylinder deactivation is more marketing than fuel efficiency miracle.