Hi,
It might be a silly question.
As UE4 uses PBR model, is it way to switch cheap blinn-phong model for some cheap effect?
Cheers,
Not right out of the box. But you could always make it yourself.
is there a way to make it without using the source code?
I would like to know this, too
Hey, there should be a way to turn on Forward rendering see Forward Shading Renderer | Unreal Engine Documentation
Might be good enough for what you need, might not be. Other than that you can create your own shading model, see for example here: Unreal Engine 4 Rendering Part 6: Adding a new Shading Model | by Matt Hoffman | Medium
although I’m afraid the second option will require some small modifications to engine (and hence some long compiles of engine code 
)
Hope it’s what you are looking for