Indiana Jones 2 and 4: why the drop in quality?

[JamesBenjamin]

I think you're going to find that people rank 2 the best, followed by 1, then 3, then 4.

Honestly, the main problem with 4 is that they left Christian mythology behind and did aliens. That and the fridge bit. If they had left out the fridge bit and had him looking for the fountain of youth instead, it would have worked much better. So to answer your questions: script issues.