Why are there different number of propeller blades on the inboard and outboard engines?
The inboard engines were of a different type, and more powerful than the outboard engines. Hence, the props had to absorb different power, so they were four-bladed for the inboard and two-bladed for the outboard engines.

Yo can see the specs of the airplane here
https://en.wikipedia.org/wiki/Junkers_G.38#Specifications_(G.38_1929)