It all depends on how important different aspects are to you. If you judge the game without taking into account anything other than level design, then overall it's about as good as the other slug games. If you take graphics into account, then it's behind the others because it does more recycling, besides X, but we won't get into that.
I think that a lot of people don't sit down and think about what's really new in part 4, though. For example, there's really nothing like the log ride in mission 3, the rope section on mission 6, or even the monkey in the other slugs. The monkey is more than a gimmick. It's actually very useful, not like the mummy. My final mission strategy always involves becoming a monkey and taking advantage of its skills. I also really like the elevator and how they used that. There's no other slug that puts you in the elevator where you're stuck on a single screen fending off soldiers. They only used it on Slug 3 for the mummies. And, the bosses are all new and some very cool. The final boss is really awesome. I think that's the best final boss on a Slug, hands down.
So, the game does reuse a lot of elements, but what it does reuse it mostly does competently and what's new is really pretty cool.
