Death. This guy would have to reduce maximum hit points and deal necrotic damage or bonus necrotic damage. His scythe acts like an ornate quarterstaff in the hands of another, but when death holds a blade of dark energy extends from one end.
War. I feel like this guy would have a vorpal great sword. Not very exciting, I know, and any natural 20 will outright kill one of your PCs so you might want to tone it down to a vicious weapon if you don't want to run the risk of instant death although an extra 7 points of damage on a natural 20 also isn't that exciting for the aspect of War. Another option I like would be to have it deal an escalating amount of damage for each successive attack against someone. Assuming he has multiple attacks, have it deal +2d6 if the target is hit twice in a row, +4d6 if hit 3x in a row during a single turn as the sword cleaves into War's opponent, hungering for their death.
Famine. I'm not really sure what kind of weapon Famine would have. For an effect, maybe something like a constitution save or gain a level of exhaustion to represent the hero wasting away. If not that, then perhaps the target is afflicted by the poisoned condition to represent it instead.
Pestilence. A dagger, pitted with rust. With just the slightest scratch the target is afflicted with some kind of horrific disease. Each time it hits, the player must make a constitution save or be instantly afflicted by one of the diseases listed for the contagion spell. Maybe have it deal bonus poison damage.