I decided to upload some parts of my schematics which might be useful for diy projects.
Simplified schematic of a 4-pin (PWM) to 3-pin Adapter:
M4 might need to be attached to some kind of heatsink, depending on how much current your fan will draw.
The circuit itself is a simplified Low-Dropout-Regulator (LDO). It may not be the cheapest solution to do it this way, but i just liked to have a solution that can be built with discrete components.