I may be wrong on this, but I think the main "speed advantage" of the P4 engine is that it does not need to create a seperate database, but starts to render instantly, using Poser's own data structures as a base. To speculate even further, that may be the reason because it's gone. If big parts of Poser were rewritten, it's likely that the P4 engine does not work with that anymore, at least not without data conversions - which would spoil the speed advantage.

Firefly can render really fast, if you set the render options to low quality. I, for my part, don't miss the P4 engine - I like Ray Tracing and using Shaders way too much. Yes, P4 can give you a fast estimate, but what's that worth if I cannot preview lighting and shaders properly? 

I know there are people out there who still render with the P4 engine, for speed reasons or because it's more crisp, or because it's not that complicated. I don't think it's some evil thought by SM to remove it, but simply a question of priorities - the new light effects only work with Firefly, including the P4 renderer would have taken more time and could have made the final product more expensive. Business decisions can be hard sometimes. 

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.