The Junkers G 38 has four propeller blades on the inboard but only two on the outboard engines.
What's the reason for this special arrangement and is there any benefit?
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)
What's interesting is the why. In essence this plane is powered by 6 identical engines, which helps with the parts commonality and maintenance.
Owing to the financial difficulties of the interwar period, the inboard bigger engines (L55) are a doubled version of the outboard ones (L8a): an inline-six configuration turned into V12. Which helps save weight and structural reinforcing, and delivers the right amount of power.
Speaking of doubling an engine, you can double a V12.
Junkers had the L2 engine in 1925, it was enlarged to become the L5 (with license from BMW). The L8 is a higher power L5. And the L55 is a double L5. It's a good engineering solution to the lack of engine diversity.
The aircraft was later upgraded to have 4 identical Jumo 204 diesel engines, the original configuration (L55 and L8) was petrol.