yeah, have fun with that 
We’ve been asking for this since 4.0, but still no dice. Hard to believe that a nearly perfect engine like UE4 cannot handle this simple little feature, limiting flexibility in a major way. I’ve explained why this is such an important thing in this thread. As you can see, Feature request has been added, but most likely Epic doesn’t consider this as a high priority 
Workarounds (none would give you perfect result like geometry decals):
- use masked material (can give it a bit of dithering, but overall will work only for decals with hard edge. Also, Z-fighting may become an issue here)
- use translucent material with “surface” translucency setting (this will “glow” and render incorrectly in shadows)
- try to position regular decals to match your surface (but that’s out of the window the second you have a curved or other complex surface).
Hopefully we will get this feature at some time soon, but hopes are not too high.