4 - Hard. I would say moderate to hard, but closer to the hard side. I had more trouble on Grim than Morbid, but they were still somewhat difficult. Grim prepped me for morbid, where I was very selective in upgrades and purchases, also the armor upgrades were helpful. I only had two sticky parts, the end of graveyard and highway to hell, however after working out a strategy and getting through them I could almost do it blindfolded. For the co-op trophy I did a brainless runthrough with a 2nd controller.
Surprisingly good game, recommended, although it did get a little tedious finishing off the last 15000 or so kills.