I come back with news, i've bought the 12-35 and i've compared it to the LX of a friend.
While it's true the LX is less practical than the Gh4, several things surprised me.
First, the stabilizer ! Better on the LX, a real pleasure !
Then, for image quality, the LX is not very sharp at f1.7, stopped down to f2.8 with 400 ISO give better result but with a bit more noise.
In fact the LX at f2.8 is sharper than the 12-35 at same aperture, quite a feat !
Otherwise, there are equal, maybe a bit sharper on the LX ! However, the frame rate is limited to 25fps on the LX :( and CINEV is missing (CineD too but i dislike it).
I hesitate to sell my 12-35 now ! The choice is really hard since the Lx has better image quality and stabilization; but the 12-35 could still be used on a future camera, not the compact...