>>11814048fun fact, metal slug 4 had the level order changed around during development. Original level order was:
- snow level (mission 3)
- city level (mission 2)
- monster level (mission 4)
- cargo ship level (mission 5)
- road level (mission 1)
- final mission
you can see the remains of this on the cargo ship boss fight, which starts with the blimp in the background shooting missiles to blow the ship up. said blimp is the mission 1 boss, so it would've made more sense for the levels to follow each other.
it's also possible they just got switched around cause the original order was the order they got made, but the order in the final game is correct.
although the snow level is much better as an introduction since it's really short and simple (unless you take the underground route, then it's three times longer and more difficult).