During the throes of one of his many deaths, GameFAQs poster Empty2002120141 stumbled upon a neat, albeit elaborate, trick for getting 99 free lives in Contra 4. First, you'll need to make sure you currently have zero lives. Next, you have to die at either the exact moment or right after you kill something that pushes your score up high enough to get an extra life. Those who've pulled off the trick suggest trying it with the first level's mini-boss. This should work across all three difficulty modes!
We're not sure if WayForward programmed this as an intentional cheat or if it's actually a game glitch, but it sure sounds helpful (if you can actually execute the maneuver).