Well, as Sahureka said, there are plenty of An-2/Nanchang Y-5s in use for the role you're talking about, and if it still works well, then why replace them?
4 engines, carrying 40 passengers or 4 tons of cargo sounds like the capabilities of an An-26 (obviously there are differences here - it has only 2 engines but can carry up to 5.5 tons of cargo). Which brings up the problem I was talking about: there aren't many roles for which North Korea needs an indigenous aircraft that big, which means the numbers they would need to buy would be small, making it uneconomical to pursue.
It must also be taken into account that Air Koryo's fleet is still operating peacetime transport and airlift missions, and could very well be mobilized in wartime (though obviously not for airdrop operations or the like). Maybe Air Koryo could buy a few more, but... Not to mention that North Korea has long relied little on air transport for domestic transport – it prefers roads and especially railways; these would also be the two main logistical methods for the North Korean military in any situation.
But I admit, suggesting "bomber" in the category is lame because 1) I forgot I was talking about a turboprop aircraft and 2) No one uses a turboprop for that mission anymore.
If you still want to replace those useless Il-28s (which I agree with), maybe you should think about giant UCAVs. So giant that it can carry one or two ALCM/ALBMs.