The P4 renderer isn't able to use a lot of the shader nodes.
Also P4 can't do ratracing so you can't do true reflections
and refractions or ray traced shadows
The pro is in certain situation the P4 renderer is faster.
The con is the P4 renderer isn't able to use a lot of the
shader nodes, or true reflections, refractions, and ray
traced shadows.
Plus Firefly gives you more control options such as
pixel samples, depth of field, motion blur, toon outline.