Hello,
I want to give you some updates.
First of all, my "normal" job has been very intense since Christmas, so I had little time and energy at the end of the day for YafaRay, that has delayed things unfortunately

On the other side, I have been focusing in two main areas of work:
* Stopped using the native Embree subdivision functionality, and I'm now exploring the possibilities with OpenSubDiv instead. The main reason for this big change is that Embree Subdivision API is... well... not nice. For example if makes so difficult to interpolate per-face varying date such UV data, but even per-vertex data is not easy to interpolate. Moreover, I could not find a decent way of generating samples from a subdivision surface in Embree

( So, doing all subdivision stuff with OpenSubDiv now is taking me a lot of time to investigate, etc. On the bright side, I hope that if I can make OpenSubDiv work properly in YafaRay, we can use all its advantages. However, there is still way to go...
* Refactoring all my v4 ugly "testing code" taking into account the changes above and also making it more "modular" so it allows different intersector kernels. For example I will use Embree and (in case Embree is not available or not desired for some reason) a much slower and limited intersector kernel called NanoRT. The main reason why I used NanoRT as a "fallback" was as a proof of concept to make sure the new YafaRay Intersector interface could handle other kernels in addition to Embree.
So, I hope my final v4 code is easier to understand and to maintain and makes more sense, allowing more expandability in the future.
I know that it's quite frustrating to see v4 is being delayed so much, but I'm really working on it. For you to get an idea of how much work I've made so far, I've made 421 commits over the last months on top of the last v3 available version. Of course, many of those commits were tests going back and forth, etc, and eventually the actual differences between v3 and v4 will not be that many, but to be honest I've not stopped much lately...

As you know this is also a learning process for me (both for coding and for YafaRay internals and render algorithms, etc), which also takes me time and effort.
In any case, things are progressing... I will keep you posted.