GH4 <-> GH3 compared at Quesababe
A high ISO comparison is presented, as well as a number of full-size pics and more 4K video footage: http://www.quesabesde.com/noticias/panasonic-lumix-gh4-analisis-muestras-video_11591
I can't tell if the high ISO test was done on RAW or jpeg. GH4 photos clearly show more detail, but I can't tell if it's a result of better pixels or less noise reduction.
They provide download links to RAW files but I can't open them in PS CS6.
They got a full production model at the last minute; first time I've heard of one being circulated.
It seems the ISO noise above 800 looks better compared to GH3, but not a huge difference anyway. Dynamic range is also a bit better ... shadows definitely show more details but again nothing to write home about.