The massive amount of ISK that can be made running missions is not your only reward. There are several other types of rewards that can be gained from missions as well. Let's take a look at what these rewards are and what they have to offer.
Besides the base pay of ISK for each mission, you will also be offered a time bonus, in which if you complete the mission within a given timeframe you will receive a bonus. This bonus usually comes in the form of more ISK but can also be implants and items of different quality.
NPC pirates have bounties placed on them by CONCORD, the police of New Eden, and when you destroy these pirates in kill missions you collect those bounties. It is the bounties that you collect that make level 4 kill missions so lucrative.