With the 1.5 RC4 I have a feeling that the light mixer consumed a large amount of RAM when is used and maybe this isn't only with RC4. Does it start from 1.4 but I'm not really sure? Is there someone with the same experience?
Yeah, LightMix can be pretty memory hungry. For example for 4K resolution you have 256 MB RAM per each LightSelect element. If you have also denoising applied to this element, its 384 MB per element during the rendering + additional 384 MB per element during denoising (768 MB total per element). This scales linearly with the number of pixels, so for 8K resolution, you have up to 3 GB per LightSelect element.
This is something I'd like to address in 1.6. I already have few ideas how to do it, now I just have to find the time to try and implement those ideas :)