In previous IJ movies, The Ark "works as Wunderwaffe" and the Grail gives immortality. The Nazis from the film want these objects as they could make their armies invincible. That's the plot.
In the upcoming IJ4 Soviets want the myserious skull for... for what? What does it do? I'm sure they don't want to simply put it into some museum in Moscow. The skull clearly looks not human (humanoid - yes, but not human). The title says "the kingdom of the crystal skull", so the whole thing is about the place where the skull comes from. The place where you can find alien lookin skull may contain other artifacts. Artifacts of similar origin.
Alien technology. Soviets want to capture ancient alien tech. This will be the surprise behind IJ4 plot.
Stargate, anyone? I liked that movie (but not what followed it), but I don't want IJ series become yet another UFO-centered flick. X-Files fans have enough content everywhere, please keep IJ movies free of such bull****, it totally ruins the style, the look&feel, the... "that elusive thing certain unique things have".
And to think that I considered Indiana Jones and the Fate of Atlantis plot to be slightly "over the top" in terms of realism...